As I understand it, a mortar is a cheap, light means of delivering a relatively large amount of explosive with good accuracy - it achieves the accuracy with a high trajectory ballistic arc, the lightness results from not having a high muzzle velocity (hence no need for a heavy barrel), nor a breech (it is a muzzle loader).
If the mortar is required to be used at the horizontal, or near horizontal, the projectile is unlikely to achieve sufficient momentum in its travel down the barrel to enable the percussion cap at the projectile's base to fire and ignite the propellant (thus, get someone to shove it down with a stick).
I can understand that modifications might be made to make a mortar into an AT weapon, but it would seem likely that this would at least involve a complete re-engineering of an already effective weapon, to make it multi-purpose.