Because that’s all that’s needed. 48 bits give you an address space of 256 terabyte. That’s a lot. You’re not going to see a system which needs more than that any time soon.
So CPU manufacturers took a shortcut. They use an instruction set which allows a full 64-bit address space, but current CPUs just only use the lower 48 bits. The alternative was wasting transistors on handling a bigger address space which wasn’t going to be needed for many years.
So once we get near the 48-bit limit, it’s just a matter of releasing CPUs that handle the full address space, but it won’t require any changes to the instruction set, and it won’t break compatibility.