Probably a cost-cutting measure. As others have said, the wider lenses get, the less they need stabilization to look sharp. The OIS barely does jack for video stabilization, most of it is software, as you can see in video taken with the new ultra wide. It's very smooth.
So all in all, it wouldn't really do too much. The ultra wide lens is mediocre optically. There's even lens distortion that for some reason, Apple doesn't fix with software. The pictures from the ultrawide have contrast and clarity turned up to extremely gritty levels - but this processing doesn't show up in video.