People don't buy paper books anymore.
I'm half joking, but that almost definitely factors in. The other main reason is that it's much shorter, at about half the size of the other novels. Also, I wouldn't be surprised if Microsoft/343i and/or the publisher are partly testing the waters for future novel releases.
That said, officially it's "digital first", which implies physical prints are at least a possibility. Considering that Stinkles has a digital short story coming soon and Staten has a digital novella later this year, I wouldn't at all be surprised to see a printed collection of the three released, possibly with additional material. i.e. Halo: Evolutions Vol. 3.