I was going to write a long essay about the relation between C and assembly but after thinking for a while, I think there is an easier way to explain it.

Stack frames are basically a (single) linked list of information about the call stack. Every frame corresponds to one function call, and says where the local variables are stored, and where the function should return after it has finished.

The head of this list is stored in a register (a scarce resource, superfast memory). So to use frame pointers, you have to spend one register, and also every function has to do some work to maintain the linked list. Two instructions worth of work when the function is enterred, and one when it exits.

The alternative to doing this explicitly is to keep track of it all implicitly which is faster but a bit more complex.