.
Range is the main reason given, but cost is a big part of it. 1 standard HIMARS missile is about $100k. 1 ATACMS is a about $1m. so while the extra range is important its not as good a bang for buck compared to the shorter range.
GLSDB on the other hand. 100 mile radius and launched from HIMARS and from reports, plentiful supply and cheap. can hit most places occupied by Russia outside of southern Crimea.
HIMARS standard has 200lbs payload
GLSDB has a 250lbs payload.
ATACMS has a 500lbs payload.
Edit, If they get GLSDB's. heres a map of where Ukraine can hit.
Vilkha MLRS can hit 120km,so they can actually hit a bit further than the 92km.
Slava Ukraini.