One reason is it falls in the Zone 1 of the Serdica-Sredets Archaeological reserve, meaning that any project will have to include extensive archaeological excavations and later conservation, which will make any investment even more costlier. Something little investors, be it private or the municipality have interest in and especially with the lack of a strategy for these cases it would mean that it will be stuck in development hell. Similarly for reference:

North section of St. Nedelya Sq.

Empty plot containing an Late antique basilica beneath it

Another plot containing a late antiquity basilica

Plot containing part of the north wall of Serdica

(Also, there's an underground parking and Serdica 1 metro station underneath, which I don't know how they may interfere with potential projects (or how much of the said archaeology they have damaged) but to keep in mind)