So 2 TELs, a radar and a ground station, that requires 4 missiles without cluster munitions, that's $6m instead of $1.5m. The $4.5m difference, which buys ~1,500 artillery shells. It's also not just cost constraints, there's production limits too.
The ATACMS variant with 300xM74 munitions travels 300km. A GMLRS-ER could carry 404 DPICM munitions or ~150 M74 munitions in the same package, so the same metrics apply to new missiles as old.
en.wikipedia.org
What about on an airfield, you can take out a SAM battery and several aircraft with one cluster warhead.