The same thing that was "wrong" with 3 worms in 1v1 (you can use the same arguments for 1vs1 and 2vs2). At least some of the reasoning can be found in the poll thread. The game length wasn't the (only) reason when tus moved to 5 worms in 1vs1, not to mention that would be a stupid reason to change the nature of the scheme.
2vs2 should be as close as possible to 1vs1 like it is in other schemes.
Elite: 4 worms in 1vs1 -> 2+2 worms in 2vs2 (equal to 1vs1 cos both players don't need to stay alive)
Roper: 1 worm in 1vs1 -> 1+1 worms in 2vs2 (since we can't play it with 1+0 worms)
WxW: 3 worms in 1vs1 -> 2+2 worms in 2vs2 (since we can't play it with 2+1 worms)
TTRR: 4 worms in 1vs1 -> 4+4 worms in 2vs2 (equal to 1vs1 cos both players need to make a time)