Probably because the Advent are much more of an attractive race to the players and community than some aliens? Think of it and compare: the Advent are some psychedelic chicks (all ships have female voices :/) and also... everyone almost is excited by psychic powers and manipulation, so here the Advent kick in, they're like the GLA for Zero Hour... Secondly, synergy is what everyone is after, knowing that you can make viable combos is something one will definitely want.
Thirdly... the Vasari look like some random aliens, running away from something they dun even know 
Judging by this century, well, chicks are the future key to maintain any game (but I dun think this is the actual reason 
)
Technically speaking... probably Vasari have more issues with "hard-coding" or whatever they need to implement, so think of this as at the restaurant. You ordered a large cake, large enough to hold you a long SoaSE match The TEC are meh... human, who be interested in them that much, when we be surrounded by 'em, lots'a dem all around daily, so they'll be the base of the cake, the Advent are what make it delicious and attractive, and... think of the Vasari as that juicy, sweetish top that awaits ya and will make ya drool
Dun worry, they might be op at start, or quite dumb with lots of buggies, but that's why subsequent patches are for 
Tip: If you are impatient to test the Vasari, just unlock them via the Dev.exe (or I think this is done so), I dunno much on modding, and I prefer to wait, but if you do know, you can change the Vasari anyway you want, will just keep you busy till da boyz come home ta throw a big party
