What eneroth3 says is the main reason. But, a side benefit is that the exported mp4 is generally better than WMV would have been, and you can immediately upload the file to YouTube, Facebook, etc. I have done tests to see what data rate is used, and the animation seems to use as much data rate as is needed for the movement that is going on. It would be nice if there was an H.265 option, but H.264 is more widely compatible.
Things have improved since I wrote this article, in that the antialiasing is greatly improved now. But for historical interest, here’s what I used to have to do to get good quality video:
https://sites.google.com/site/sagesuwiki/tutorials/tips/exporting-animation-for-web