Higher value, higher priority, well defended targets for SRBMs I guess?
Kimscanders now routinely strike Kiev, and almost certainly DPRK officers are very well engagement in empirical calculations that make it happen.
MLRS absolutely can hit such targets(and they can simply burn through AA stocks of most nations), but in a grand scheme of things it's less reliable weapon per round v defended target, here and now.
On the other hand, SRBMs are expensive, proper missiles, limited by production complexity of their solid fuel rocket engines.
Ones that evolved from MLRS can and do fly on pressed dry grass simple, mass-produced "artillery" chemistry.