The advantage MP3 had is that everyone respected the container format for the most part too, and while there was some wonkiness around VBR for a bit, generally a device that could play MP3s could play the music in the file regardless of ID3v1 or v2 tags and such.

DivX and Xvid, along with Quicktime, and many other encoders technically do use MPEG4 video, but they by default choose to wrap it in an non MPEG (mp4) wrapper, causing playback issues on devices. For example, out of the gate the PS3 would play MPEG4 files just fine, but if you gave it a DivX file (either AVI or DivX container), it just said unsupported video format. Now that Sony paid for a DivX license, it now plays.

Even the base MPEG4 has a ways to go to becoming the MP3 of video. Can Windows Media Player even play back MPEG4 files yet without an external codec? Also, I do see the adoption of H.264 into Flash 9 as helping quite a bit, as it should hopefully get the encoders a bit more friendly.