Why would Dalish elves choose to live in the Free Marches?

Good word that: "relative". Trouble is they have never really said how big the distances are across Thedas. Well, back in DAO, Alistair said something about it being a thousand miles (or was it more than that?) to Weishauppt, which is why we couldn't get a message to there quickly (the Grey Wardens apparently not having access to sending stones or even rapid raven post). Still, if we assume a thousand miles, that is roughly the distance from London, England to Madrid, Spain. To be honest that isn't a lot, so may be I am remembering that wrong or the writers just have no idea about distance.

The Freemarches are pretty extensive, so it depends on where you are in them, to how close you are to the Tevinter border. Over in the west, Hasmal is right on the border so escaped slaves and slave hunters are often found around here but on the Tevinter side is the Silent Plains area, which is pretty inhospitable and well away from the main centres of civilisation around the Nocen Sea, so I imagine most of the slaves are fleeing from the settlements of Perivantium and the Hundred Pillars on the east side of the Imperium. I think you also need to draw the distinction between slave hunters, who are specifically tracking down escaped slaves, and the slave traders. The latter seem to operate with impunity across southern Thedas, as evidenced by their presence in Kirkwall. As Xerrai says, though, they probably find it easier to kidnap poor people from cities or trick refugees (as Isabella highlights) than go out in the wilds and attempt to locate and capture Dalish. If a clan should fall foul of their human neighbours, I wouldn't put it past the latter to spare the children so they can sell them on to a passing trader. So I wouldn't think the Freemarches are any more dangerous than anywhere else in that respect.   There are plenty of remote, wild bits in the Freemarches.   There is the Planescene Forest, Wildervale (the clue is in the name) and the length of the Vimmark Mountains.  It also made sense that clans, like our own, would keep to the borders of one city state and another so they could constantly cross over and make use of the rivalries between them to avoid attack.   I seem to recall the the Dalish in Antiva settled in the region of the Weyrs, so may be they benefited from the reputation and protection afforded the place by Yavana.  I imagine one Witch of the Wilds would be pretty much the same as another to them.   Wasn't there also rumoured to be one in the Planescene Forest?   May be it was Flemeth who first suggested the move across the Waking Sea to one clan and the word spread.

There are some oddities to the spread of the Dalish that I may have mentioned before. When the Dales fell it made sense that the Keepers and their charges, the families of the Emerald Knights, would flee eastwards over the border into Alamaari lands that were not under the thumb of Orlais. If you take into account the Frostback Mountains, the Korcari Wilds and the Brecilian Forest, there is a fair bit a territory that they could lose themselves in. To flee in the opposite direction would have taken them directly into enemy territory at a time it would have been swarming with hostile citizens, whether soldiers or not. I can also appreciate that over the succeeding 800 years their numbers may have increased sufficiently that they thought it wiser for some of them to migrate to the other side of the Waking Sea. What I find hard to understand about this, and the Sabrae clan subsequently doing the same, is how they found people willing to transport them and, if it was simply a matter of money, how they acquired enough to buy them off? Nevertheless, leaving that aside, clearly the nearest suitable territory would be the Freemarches, particularly the eastern provinces and they do seem to have spread northwards here, reaching as far as Antiva and even Rivain. Some apparently even crossed to the island of Llomerryn and have a semi-permanent settlement there. Presumably they get on well with the pirates.

One place I was surprised to find them in Tevinter Nights was the Arlathan Forest. Since that was the place that Tevinter originally captured them, it would seem a dodgy move to return there, particularly as I would have thought Tevinter would have maintained a watch on the place to prevent this. However, again in Tevinter Nights, it would seem the forest has its own defenses that should keep humans out, although in that case I wonder that they ever managed to invade there back in ancient times. Still, it would appear the clans were probably settled on the far side of the forest, furthest from Tevinter. It was also odd that there was that other clan actually in Tevinter territory, in contradiction of what Merrill said in DA2 about the Dalish keeping away from Tevinter for obvious reasons. Didn't we also say the same to Dorian in DAI? So I will be interested to learn exactly where these clans now circulating in the vicinity of Arlathan Forest originated. Are they clans that migrated north some years ago and then lost touch with their southern kinsmen? Are they clans that have traveled north more recently as a result of the influence of a certain Dreamer on the dreams of their leaders encouraging them to do so? Or, at least in the case of Strife's clan, are they not really true Dalish at all but a separate faction of elves?

EDIT: I had an idea about the time period when the Dalish first crossed the Waking Sea.  It occurred to me it wouldn't have been before the 4th Blight because otherwise they would have been wiped out by the darkspawn who swarmed across Antiva and the Freemarches.  Then I noticed that Calenhad didn't unite Ferelden under his rule until after the 4th Blight.   He also threw in his lot with the Chantry and backed the imposition of the Circles on the whole of Ferelden, with associated Templars.   That might well have been the catalyst that prompted the clans to split up, with one group crossing the Waking Sea in order to give themselves some distance from the new situation should the Templars start going after their Keepers with the backing of the monarch.   Then not long after this, the Qunari appeared on the scene, so the humans of Antiva and the Freemarches had other things to worry about than a few Dalish clans.   The Weyrs, though, would have made a good refuge for the Dalish because Yavana opposed the Qunari and wasn't afraid to use her dragons to prove it.

As you often say, it is better not to get too attached to concepts in this series. That is, beyond the very basics. 

But I must ask, when was it mentioned that the Sabrae clan didn't know how to contact other clans upon reaching the Freemarches?

And while we're at it, I'd like to add a completely new question: What determines each clan's atittude towards humans? It seems to me that each clan has a certain... uniformity to their level of distrust. Most people in the Sabrae clan have a moderate attitude, most in Lavellan appear to have an amicable attitude and those of clan Virnehn are generally extreme.