Blizzard Entertainment once considered only allowing six-man stacks for competitive queues for their popular game, Overwatch. While this could have provoked interesting gameplay, devs saw that this may create some problems.
During an AMA on Reddit, Blizzard developers provided some insight on different questions, including the above mentioned topic.
Scott Mercer, the Principal Game Designer at Blizzard, gave a couple of reasons as to why this option was not viable. The first being that having a competitive game mode with six-man stacks would not be fully inclusive. Limiting a team to just one hero could prevent players who may not play that hero from participating and may even further discourage them from playing the game mode altogether.
The second reason that Mercer cites is a balance issue. The Principal Game Designer says "Six-player only teams solved some matchmaking problems... but would create others such as making it more difficult to find fair matches."
Mercer then explained that besides the balancing issue that this mode may present, he says it could also be a logistical issue. 'Let's face it, queuing solo is just really convenient." In that respect, he's right. Finding a time to put together a team of perhaps friends is much more inconvenient than just hitting a button and being matched into a game.
The last reason Mercer cites is that solo queue is just too popular not to have. "If we didn't have a solo queue the first thing people would have asked when we introduced competitive play would have been, 'That's great, but where's a solo queue?'
Although there may have been an audience for this type of game mode, devs felt like it was not the right time or place for an only six-man stack. However, at the end of the post, Mercer says some form a six-man stack game mode may become available in the future. Read the full post here.