Even the Atari BASIC cartridge/ROM was supposed to be MS BASIC[0] but it didn't fit in the 8K ROM leaving any room for additional interfacing with the machine. MS BASIC was still offered separately.

I'm glad for the Atari version. Although it was less powerful, it was enough taking less space and running faster. An Integer Basic like on the Apple would have been great to have too.

https://en.wikipedia.org/wiki/Atari_Microsoft_BASIC