They need to keep them digital, but I think the biggest factor in movies being a success on 3DS have to do with consumer adoption.
When UMD movies came out, DVD was already entrenched, and people generally didn't want to spend the same amount of money for a movie twice just to have it portable. The other problem was that Sony never followed up on their initial promise to spread the UMD format beyond PSP, and have dedicated UMD movie players on the market. The rise of digital formats for mobile use probably played a large part in that, but the problem remained; UMD was just as expensive as DVD, but more limited (less space, less extras, PSP only).
The key difference with 3DS imo, is that 3DTV isn't already adopted by the mainstream (and doesn't look like it will be anytime soon), so there's already immediate inherent added value in a 3DS movie release that didn't exist for UMD. This isn't limited to 3DS either, but really any 3D capable mobile platform (like those Sharp phones). And now it looks like Hollywood is catching on, and seeing mobile 3D formats as possibly a better consumer alternative to 3DTV at this stage...
