Because it's a Tor policy? Brandon Sanderson's ebooks are DRM-free because his publisher is Tor.
But it's also useful to point out that it isn't just Tor. Baen, best known for military scifi, has had a DRM-free policy for slightly longer than Tor. (Not just that, but the Baen Free Library is a really cool approach to ebooks as well, with DRM free copies of some of their most out-of-print/hard-to-print books and also the first book or two in nearly every series that they publish for a "try before you buy". Some of their hardcovers have even included CDs of sections of the Baen Free Library over the years.)
Baen did it first but Tor did it louder in that Tor's parent Macmillan went to bat for Tor and few other brands in a big lawsuit with Amazon that Amazon was applying their DRM whether the publisher wanted that or not because it was a lock-in moat for Amazon, which led to why there is now a required "This book is DRM free at the request of the publisher" acknowledgment on kindle copies of most Tor books (and a few other publishers).
- Brandon Sanderson's books are actually relatively inexpensive, despite their popularity
- Brandon sanderson ebooks are available without DRM. Interestingly, this is actually more common for fantasy and SF than other genres.
Other books are more expensive and more likely to be locked behind DRM for digital books.