I seem to remember the reason having to do with them needing to sell it the way they are now is so that they can offer a demo. If you buy directly from Runic, the only DRM is an online activation, you get ten of those, and if you run out for some weird reason, they're willing to give you more. Plus, they've stated that if anything happens to the company, they'll patch out the DRM.
So really, while more platforms is always cool, and is potentially (but not definitely) more profitable for them, it's not like it's necessary.
As my footsteps on the roadway click
There's a place I've yet to be.
My heart is always homesick
For a place I'll never see.