From what I'm reading, it has to do with the lack of an actual separate SD Card partition with FAT32. UMS requires a block device so they would have to give use direct full access to the entire file tree. Moreover, the entire file tree is EXT4 and not FAT32 so even if it was exposed you wouldn't be able to read it in Windows without a 3rd party driver.
They're moving swiftly away from SD Card support which was one of the big reasons I supported Android over IOS. I haven't found any app which worked yet. I ultimately had to use FTP to transfer the files which was absurdly slow by comparison. This, coupled with the crappy AMOLED display and burn-in concerns, has me thinking I may send this Moto X back.