Wait, we have? The equipment stat only said "mortar-carrying and artillery-carrying variants are already being considered", but I don't think we actually do anything more about it. The description for SPG/SPAA Systems also seem to implied that it has been skiped in favour of proposed modular system (that we don't have time to build) instead.
It's one of those 'natural' variants that didnt need a big procurement program, since mortars arent all that complex.