The answer to "why not always put it on the stack" is "because a lot of functions are leaf functions and so always writing it to the stack is making every function pay the memory access hit rather than just the ones that need it". RISC-ish architectures tend to have enough registers that dedicating one to a link pointer isn't a big deal (and once you do spill it to the stack you can use the link register as a temporary register anyway).
Some very early CPU architectures didn't actually support either putting the return address in a register or on the stack. For instance, on the PDP-8 (https://en.wikipedia.org/wiki/PDP-8#Subroutines) the JMS instruction writes the return address to the first word of the subroutine it's about to call (and the actual subroutine entry point is just after that), which meant it didn't conveniently support recursion. It wasn't alone in that either -- I think that it just wasn't quite appreciated how important recursion/reentrancy was back in the early 60s when these ISAs were designed.
Some very early CPU architectures didn't actually support either putting the return address in a register or on the stack. For instance, on the PDP-8 (https://en.wikipedia.org/wiki/PDP-8#Subroutines) the JMS instruction writes the return address to the first word of the subroutine it's about to call (and the actual subroutine entry point is just after that), which meant it didn't conveniently support recursion. It wasn't alone in that either -- I think that it just wasn't quite appreciated how important recursion/reentrancy was back in the early 60s when these ISAs were designed.