Probably because the iPad isn't powerful enough to convert raw data, wrong tool. So you have to build smart previews which are partially rendered but as you point out, not fully raw data.
The idea of having to render the raws on a real desktop, just to get them over to the iPad kind of illustrates the weakness of this current workflow path.
Even if you can't edit RAW files directly on your iPad... they should just allow for an easy way to sync them back and reconnect them in the desktop... Right now everything you do on Mobile is worthless, you have to start all over again with the RAW files back in the desktop!
You should be able to just import your RAW files and they should reconnect to the edited JPEGs, with the develop settings from the mobile app. Without this, Lightroom mobile is just a too for people shooting only JPEGs.
I just got back form vacations and tried the mobile version for the first time, and I loved it, but I feel I'm losing something with only correcting Jpegs instead of RAW.