Why not 32-bit addressable?

PC were 32-bit orientated prior to the introduction of 64-bit, so why did IBM upgrade from 24 to 31-bit address ranges.

The issue related to the fact that when System/360 was introduced in 1964 the view was that 16 Megabytes of memory would be adequate, and the high order (left-most) bit of any 4-byte area holding an address was reserved for other purposes as indicated opposite.

One use of the high order bit, was that if it was on, i,e. set to 1, it indicated that this address field was the last in a list of addresses.