The IBM 360 was constrained to a max of 64k memory.
In the company I worked for, we had 5 370's, the biggest was 512k until 78 when they upgraded to a meg.
I think they did it to support TCAM the predecessor to VTAM.
The IBM 370 was constrained to 268 meg until around 84 when they came out with XA.
That was a PITA due to all the software that had used the upper 4 bits of the address to pass flags.