Honestly I think most of the reasons at this point are 1) 'we need more dev/QA people or hours to work on this' or 2) 'staff need to sit and figure out exactly how this should be implemented for it to not be a disaster'. A looooot of stuff is (1) - beta entry page, new API, finishing moving off BML, better interface to the image upload system. The biggest thing I can think of in (2) is the possibility of a (opt-in) kudos/like-style system.

(I am not staff, this is not official calls, but I'm a dev who's done a lot of work on the site and on a lot of those previously-mentioned dev projects XD)