1. they are way too expensive
2. by the time you charged them, there are not territories left around them to fire at
3. they miss so often, that even if you fired mortars nonstop, you could probably finish the entire level without hitting a single shot
Number 1, of course, is the biggest problem with mortars as currently implemented. In a recent level I encountered a mortar that required 3.36B armies to load. The largest territory within firing range was 1.2B in size. For them to have any conceivable use they need to have their costs adjusted so that they are potentially advantageous.
Number 2 I regard as part of the tradeoff with mortars, if the first point were fixed. You could wait for an advantage, or carry on taking territories now but less overall efficiently. That is, at least in theory, an interesting decision to make.
Number 3 is a design feature that I don't like. I've commented before that the randomness feels very out of place with the rest of the game. Diminishing damage as the targets get further away makes sense to me.
All of the above being affectable by techs would make a lot of sense. I hope that there are plans for this in the future.