Andromeda - Where is everyone else?

I predicted that the final five cylons had come from earth, and that earth was the ancient cylon homeworld (i.e. the cylons from Kobol rather than the cylons we know) rather than salvation for humanity. This was back when 4 of the 5 final  cylons had just been revealed, a little after the temple of five episode. 
I thought this was likely because of the 'cycle that repeats itself through time' and also why did the 12 tribes go to the colonies while the 13th tribe went somewhere else, unless they were cylon? About 10 episodes later, it was shown I was right. 
Even if we accept your criteria and exclude codex races, there are still at least a dozen or more races in ME1 and three in ME:A that we know of. So not really tendentious at all. Did the writers not write any more races for andromeda in this game? And if so, what is their rationale for it? Why write a single cluster and not have anyone outside the cluster be able to interact with it, if the case is we only find 3 races?

Yes, in the BSG tv series, it was the 5th cylon Ellen that said the cylons from Kobol made the temple of 5 before they went to earth and in the episode where they landed on earth, they confirmed that the remains were cylon.
They weren't reinvented there because no humans were in the 13th tribe.  Its been a long time now but I know someone pointed it out in the tv series that the cylons on earth were from Kobol. 

I understand your point, but I think its a bit of a moot point. Someone else pointed this out previously in the thead but if we only look at races that are non-codex is ME1, there are still at least over a dozen races in ME1 compared with ME:A which has 3. So it doesn't matter whether you just have ME1 races or include all races, the point is still valid. 

I think one of the writers or the producer has just said a few days ago on twitter or somewhere that andromeda doesn't go through extinction events like the reaper cycles such as happened in the milky way, so I doubt thats the case right now.
When people suggest that anromeda races haven't advanced as far as the milky way races despite the fact that technological development was curtailed for all races in the MW for billions of years by the reapers, it suggests that the MW is special for no reason and the poor andromeden races are somehow worse off even though all of MW galactic civilisation and its technology wascompletely wiped out every 50 millennia ( I don't know how much worse off you can get than that).
Also the over abundance of planets in the MW alone, a galaxy half the andromedas size also suggests that the MW is magic and defies probability. It doesn't seem very convincing. 

If the producer did say that then excellent because that would have been lazy. As far as technological advancement in Andromeda is concerned we have no idea how old/young the Angara and Kett are unless this information has been disseminated (I really don't know and haven't looked), so gauging where they "should" be technologically can't really be done. And remember, it was only because of the Prothean ruins left on Mars that humanity was even propelled into the future in the first place. Who knows how long it would have taken humanity to progress had the Protheans decided the Sol system wasn't interesting enough to visit. We also haven't seen any proof definitively that the races of Andromeda have access to something as commonplace as eezo (which does make the milky way special for obvious reasons) or their own equivalent. Also there's a good possibility that some of the races of Andromeda didn't go the way of a reaper-like extinction, but rather the way of nuclear holocausts akin to a few of the Milky Way species. An interesting perspective I think we should also take into consideration is this, Leviathan->Catalyst->Reapers->Relays. It literally took creatures with near god-level intellect creating an even more superior AI that synthesized all life in the MW in order to create the mass relays and therefore a reasonable means of transport throughout the Milky Way. If it took that much just to create a way of travel superior to FTL, Andromeda having millions of years of a head start or not doesn't at all mean that they would or could have managed the same or an equivalent feat. This also shouldn't be a knock against them because there are just too many variables to consider in terms of them making progress that can't just be explained away with "well they had x amount of time to figure it out". I don't think the size differences between the MW and Andromeda really suggest anything apart from probability. Yes, a larger galaxy has a better chance of possessing more species/resource diversity and the corresponding technology over time, but it isn't a given even if we're talking about a video game.