There could be a lot of reasons. There are 4 on the market and 2 are under contract. In this market, that is pretty good. Are these for investors or owner occupants? If too many investors own, then it is difficult to get funding. I've been in one of them with a client who wanted many more windows and light, nor did she want to walk up a flight of stairs every time.
It is a buyer's market so people can be choosy. Not everyone wants a condo that is in an old block building, even when it has many bells and whistles. Finally, it all comes down to price. At the right price, they will sell quickly. Sorry I can't tell you more definitively.