Comment by lapsed_lisper

IDK how much this matters, but the Common Lisp standard doesn't mandate tail call elimination. So although many implementations offer it (usually as a compiler thing), it's conceptually an implementation-dependent detail that borders on a language extension: if your code actually depends on TCE for correct execution, that code might exhaust the stack under ordinary interpreter/compiler settings, and differently across different implementations. So for Common Lisp, if you want to use standardized language features standardly, it's quite reasonable to reach for iteration constructs rather than tail recursion.