There have been a few groups that used h264 in avi earlier, when h264 was just gaining more popularity. Back then there were always people who complained their players or player - splitter combinations weren't able to handle it. I doubt such problems exist anymore (except for those who only ever watched Naruto and Bleach and stuck to Windows Media Player or some pesky stand-alone players).

Avi is an old and limited container, and just like groups moved to the new h264 encoding they mostly moved to new containers as well. It makes sense even if they decided not to use mkv's (or mp4's) better features.