nt.number theory - Why is the largest signed 32 bit integer prime?

This may be subjective, but does anyone have any insight into why this is the case? This struck me while considering that it's also the eigth Mersenne prime (2^31-1=2147483647).

I'm now wondering why this might be the case.

UPDATE:
It's been pointed out that the relationship doesn't necessarily hold for larger storage classes, e.g., 2^63 - 1 is not prime.