It's not about the Trios event itself, but it's implications.
To me, the integrity of the Invitational event is paramount. In a Trios event, it is possible that two excellent players can drag along a third player to victory who may not otherwise be "invited". Also, in a single event, Best General(s) and Player's Choice(s) would net 6 automatic bids out of 32 available spots. That's huge. Then what if two players on a team were or weren't already invited... Trios basically created a bunch of "what if?" headaches.