Checking with the KVS Availability Tool & looking at random dates, I see precious little D bucket availability on this flight. (Earliest I came across was mid-week at the end of June, but I'm just poking about at a few dates.) OTOH, there seemed to plenty of (limited) A bucket availability.

It looks as if you need to reserve a long way out or think about paying for a bit of pampering if you want to pass by CPT on this trip. (PS: It's worth it, IMHO.)