Official reason for killing 2v2?

There are three main reasons.

One, the 2s comp tended to reward sticking the most abilities into one class. Really well-rounded classes with lots of offense and defense tended to dominate simply because you have fewer people in which to package everything you need.

Two, there has to be a point at which you balance healers. If a healer can keep themselves and one other player alive, then they are hopeless in 3s. If a healer can keep themselves and two other players alive, then they are overpowered in 2s. (By the time you get to 5s, we think other factors come into play so it matters less that the dps to healer ratio is the same.)

Three, we just think the larger matches are more fun overall and we think we can offer a better game experience by focusing on them. If you just love 2s, well, they're still around. They just don't offer the best rewards so perhaps the players most focused on balance won't obsess with two vs. two balance.