Sounds like a classic case of Sega rushing out yet another game... At least for the original US/JP version. It wasn't that Sega added Super Monaco GP but rather disabled the game from the menu. The data is already on the disc along with the menu icon and palettes. Along with the menu music being too quiet, I wonder whether Sega did release a beta version as a final. Someone should really disassemble the code to see why SMGP got disabled (and whether Golden Axe can get fixed for 2 Players).

Since the Mega CD had a late release in PAL regions, it meant that the finished game was able to get released and an excuse to re-release the game in the US.