Did Grant just stop having anything to do with the show?
Yeah because he was bored with writing it and wanted to try things out on his own. Rob and Doug actually thought about splitting several years before that but kept going due to Red Dwarfs success.
Also Rob hasn't been very successful on his own. He doesn't really seem to know exactly what he wants to do. He's written some novels, done stand up, the odd TV show but nothing major.
Rob and Doug have sort of made up and do talk to each other now and again but there was supposedly some acrimony over the split. The Red Dwarf USA fiasco didn't help things either.