DirecTV & Showtime/Viacom have always seemed to have a cozy relationship whereas with HBO, not so much.
It took us *years* to finally get HBO Zone--and we still don't have HBO Comedy (drop HBO Family West if you have to! lol)---yet we have all the Showtimes (save Showtime Family which I believe they reserve as a "cable exclusive") and the Starz channels. In addition, Showtime and Starz give DIrecTV special discounts--which is why you can always get a few free months of either, while HBO is unavailable for any deals.
IIRC Showtime is even available as a "flat rate" to D*, which is why they push it so hard!
--Nat