MKV is a lot better for the people making the releases. It supports H.264 natively, has proper softsub support, file attachments, chapters, segment linking... And it has an awesome toolkit to work with it, MKVToolNix. And the format is completely open and free to use.
And when the people making releases have better tools to work it, it gives us faster results and/or better quality. It's a win-win situation for everyone.
Instead, people who stick to AVIs and XviD releases are slowing things down. Doing an additional lower-quality XviD encode takes time (believe me, any group that does HD releases does HD versions before the SD versions), and basically makes releases slower. Hardsubbed releases are also harder to fix afterwards if you notice an error, since you have to re-encode the whole thing. AVI also has all kinds of other retarted limits which just makes things harder for the encoders.