Perhaps it is just an expensive license (plus paying the designer) so it's a matter or demand vs production cost. If demand was the only issue, TBBT has been heavily discounted and has even been given the SAH extra VIP points bump. This one hasn't really been available below retail in the US without coupons/GCs etc.
Side theory: Sets with less demand like TBBT are allowed to linger since they are owned by WBs, which has a lot more properties that Lego wishes to continue producing, while the BBC doesn't. Lego used their license to make a few Doctors, a companion, Cybermen, a Tardis, and 2 versions of Dalek. They don't see much room for growth and recognize they have to retire something in the Ideas line with new sets on the horizon. Wall-e (Disney) Ecto-1 (GBHQ pairing/good seller), Maze (new), and TBBT (see above) all have reasons to stick around. This one probably doesn't.
Renew the license with no new sets to make, or just do a Scrooge McDuck dive into your 1-year Doctor Who profits and start pushing that yellow car thing? Sorry for rambling, just taking you guys with me through my thought exercise on this one...