MCGA was basically a cut down VGA on the motherboard of the low end 8086 / ISA PS/2s, and both chipsets were introduced at the same time. Next-to-no-one would want a not-quite-VGA that lacks high res colour or EGA modes, so nearly nobody bothered cloning it, only the higher end option. The only known clone is in a couple of Epson models, and like the PS/2s it’s a motherboard integrated chip, not an expansion board.

Because it was the only "standard" 256-color mode back in the day, supported by both mcga and vga. MCGA cards had just enough ram to support this mode (64kb) while vga cards were a strict superset and had a full 256k of ram.