My impression over the years has been the following regarding new teams being added. Also please note, when I say "the community" I don't mean 100% of the community, I just mean the push came from the fans and players, not GW or the BBRC:
Slann- A team with a unique playstyle and some 2nd Edition history. Not much community push to make the team, but Galak in particular (?) really did like them, and the team plays so different that it "Stuck"
Chaos Pact- The Chaos All-Stars, the most famous team in the game, were a Chaos Pact team, and there have been attempts to reproduce them since this edition was 20 minutes old

All attempts did pretty poorly, but I think the community really wanted this one, and eventually an acceptable team was found.
Underworld- It shocked me, but there were a lot of coaches who really wanted this team. I don't recall any members of the BBRC having this as a pet team, I think the community as a whole wanted this one, and the rules were straightforward.
Pro Elves- As mentioned earlier, this was GW driven to replace the High Elves/ sell new minis/ put off BB fans. I honestly didn't play Blood Bowl much from 2000 to 2003, so I missed where this team came from, it's all been hear-say.
Khemri- GW driven in an attempt to "split" the undead. I'm not sure how popular they have ever been, and they have been a thorn in the side of many players and the BBRC for years. However, JJ wins!
Necromantic- GW driven attempt to "split" the undead. The team is fairly popular, if considered to be a weaker version of the Undead team.
Vampire- GW introduced, and the community seemed to like this team despite it being nearly unplayable/ completely random in previous editions with OFAB. It was only community support, the models being cast, and the BBRC working that made this team a success (I love them.)
Ogres- Actually, I missed this one completely. I don't know who's idea it was, but there were models cast and people play them. Their rules have been heavily changed, and currently you can't even buy BB snotlings!