Because they use custom-DRM, that makes it not feasible. Without the free-demo password, which they change every release to something different (to avoid pattern matching software figuring it out), you can't log in. And the legitimate way to login requires you to have a specific file, that has a unique footprint, that is easily traceable by the dev to figure out who leaked it.
Someone tried to hack it, but failed. So its either wait 5 years for a steam early access release, get gouged each month for service, or wait till the free builds. Personally, I'd rather wait for the steam build, and haven't actually played it since release 06.
Plus the available download sites aren't feasible. The game hit over 5gb, so Mega won't let you download it without paying or having a VPN. AnonFiles always gives me a virus detected warning by my Antivirus. And the remaining two, cap your download speed to like 100 kilobytes per second (100kb/s), when I have a 300 megabit connection (or roughly 38 mb/s).