Packing density of MOSFET
MOSFETs are easier to scale down simply using W/L ratio. So on a given area, more MOSFET memory cells can be accomodated, compared to BJT. Also MOSFETs have low static power consumption, when holding a logic state in a memory cell. One more point is that, from the fabrication point of view, MOSFETs fabrication process is less complex, compared to BJT. So MOSFETs are a better choice for memories.