Cannons
Pistoliers
Outriders
Great Swords
IC knights.
Cannons: serve a different purpose than mortars. Cannon cracks one big nut, while the mortar is for pest control.
Pistoliers Ourtiders: are mobile and shooty, but do not fare well against shooty enemies. The mortar, in contrast, is static, but is can suck up a great deal of incoming fire as most shooting willhit the machine and not the crew. At T6 the mortar can suck up a lot of shooting.
Greatswords: Expensive, compared to the mortar, and again, both serve very different purposes. The GS are a tarpit unit with a decent combat capacity, while the mortar is used to deal with threats from a distance. You can use the mortar to whittle that unit of swordmasters down before they hit your lines at full strength. Using the greatswords against those swordmasters will *not* be as effective.
IC knights: Again expensive, and again, completely different purposes. You've got to invest heavily in them to make it worthwhile, often building your army around this particular unit. But IC's will still get thier asses kicked against a lot of the nasties out there, and they can't kill enemies at a distance like the mortar.
And don't forget that the mortar is actually a pretty effective sniping weapon in a pinch. Str 6 D3 wounds can knock out monsters, chariots, war machines and the like.