You'll find people much better suited to explain this on HN, but I'd guess it's about efficiency. CL was defined long ago and one of the goals was for it to be

You'll find people much better suited to explain this on HN, but I'd guess it's about efficiency. CL was defined long ago and one of the goals was for it to be fast. It really is, but there is a pervasive mutability in most of it because of this, with functions like nreverse, rplca, nconc and so on.