Why is Jaal's romance so popular?1

Well, techno assistance was what I meant. We are in the world where Angara themselves are basically space homunculus, and where andromedian native technology incorporates genetic structure into kett super genome, and where our own MW Asari procreate with everything that has biological material and sentience. honestly, this is not an avenue I do want them to pursue any further, but they can. Alternatively, a lose Angaran extended family structure does allow for intergration of F!Ryder as an auxiliary "mother" in Ama Dara clan and nurturing a multitude of Jaal's nieces and nephews as well as three and five times removed.... 

He has literally the most content for a female ryder to romance a man. So if you want your ryder to fuck a dude you have Jaal or Reyes. I haven't done Liam's, but from what I hear his is pretty short too. So really Jaal is the most content heavy. Reyes is one romance I love too, but his content is shorter.

Apparently many people have a thing for shaved cat balls

Well, I went into the story completely spoiler-free and had no idea Jaal was even romanceable for most of the game. I didn't know what the people holding hands icon was supposed to mean (friendship?) so I picked it. Then he sends that email calling her "dearest one" and I was a little confused, especially because he wasn't talking to her after blowing up the kett facility on Voeld. When he was willing to talk to her again, she was already or had already been involved with Liam, Peebee, Reyes and Keri... so I figured why not? And she romanced him too when the hearts became available. It was interesting and kind of cute as a romance.

I was more into Jaal's hype than I was into Jaal tbqh.

I don't know... You are right, of course, that tastes are different and you do not have to understand why one likes apples and another - oranges. But there is such thing as objectivity. For example, I do not like any romance option in DAI, but played them all and can honestly say that objectively Solas - is the best romance in the game. Professional writers can point details that make it so, but from pov of simple gamer you have there mystery, passion, relevance to the story, personal agenda (that also evokes emotions) and simply good writing. While, say, IB - irrelevant comical fling consisted of several gags with moralization in between.

You can head-canon any deep means for every word of any romance and read "between the lines" or analyze face and voice expression of your chosen LI, to rationalize that choice, but things like plot-relevance or emotion stirring deeds remain objective - they are either there or not.

In our case, Jaal is everywhere, relevant to practically every event and main plot, unusual (not the boring human male), romantic hero (in all ways -  a Resistance fighter and emotional being) and written with some humor and tenderness in it's dialogs. I'd say Reyes rivals him in every aspect while being the opposite in some of them - for darker flavor. While all Liam can bring is ... well... there has to be something!... he cares about you? Something like that. But take him away and not a single mission or main plot-line will change.

I'm pretty sure people loved Jaal before MEA was even released. But, honestly, I don't know. He's not my favorite squadmate and I wouldn't be interested in romancing him even if he were an option for Scott.

I can see how Solas could hit all the right romantic buttons for some, and I think Jaal behaves in much the same way, but neither really works for me. I'm after people like Alistair, Cullen, Reyes and Kaidan.  However, BioWare finds the Solas/Jaal type to be what the average female wants and they aren't wrong.  I just don't fit the category.

Never had any interest in Zevran.  Romanced him because he was the option and I didn't like it at all.

For the record, I don't hate Liara.  I see her as a really good friend to Shepard, probably suffering from unrequited love, but will to do anything for him or her.  I later began to see her differently, taking a look at how she ignored Shepard almost getting hurt during LotSB (building explosion) because she's so singleminded about saving people she cares about.  Shepard has already been ticked off as "safe" at this point so she focuses on Feron.  Only after he's safe does she become "human" (so to speak) again.  Then, in ME3, she enshrines Shepard in her little pet project, as well as offers a special mind melding gift toward the very end, just to share some comfort should they not survive.  She's really very sweet.  It just took me some time to notice.

In any case, I don't see Zevran or Liara as being anything like Solas or Jaal.

Not a worry of mine that things are derailed.  I also have a tendency to romance humans.  You may have noticed.  Hence, Zevran, Solas, Jaal, Fenris, Iron Bull, Garrus and Thane mostly don't hold much appeal to me.  I guess I look for what I would want in a man, and the pointy ears, scaly skin, big horns and plated skin just don't fit for me.  Lol

I think it has more to do with their popularity among the female gamer fanbase (among their relevance to the plot), tbh. The same could be said for Alistair, but Solas is more recent.

Edit: Misunderstood the context of your post.  But, I still think that's why people tend to group them together.