There were multiple reasons.

Simons' BASIC was still interpreted. If you wanted graphics and performance, you used Assembler.

Many/most people were using a disk speed-up cartridge like Epyx's Fast Load. You could only use one cartridge at a time. If they placed Simons' BASIC in the C64 ROM, that could have made it less compatible with existing software.

They didn't want to pay the licensing cost for an application that only a minority of C64 owners would have wanted.