They are primarily because the Shep/Weir fans are all over Elizabeth as the wonderfullest, bestest character evah. I haven't met a Shweir that didn't think that Elizabeth was the bees knees. If Elizabeth is someone's favourite character, chances are, they're a Shep/Weir.
By contrast, most of the John/Teyla fans are all over John as the wonderfullest, bestest character evah. Teyla isn't important - she's pretty much the female stand-in for the pairing, displayed in the way that most John/Teyla writers don't actually write Teyla... They write John with a female lover that goes by the name of Teyla but isn't actually Teyla.
One of the reasons that a lot of John-fans ended up in John/Teyla rather than Shep/Weir (or Shep/McKay) is because they don't have to contend against the Teyla-fans - because there are so few of them. The Elizabeth-fans and Rodney-fans are numerous, plentiful, and, above all, fannish. Which means the tide of fandom goes their way - particularly in relationship stories.
I will bet anyone a 20,000 word fic that you'll find that most John/Teyla fans are way more into John than they're into Teyla - and not just because John has more screen-time. Scrape down and you'll find that the chiefest offenders of Teyla-characterisation are those who are in love with John but haven't descended into writing themselves as OCs to be in a relationship with him.
