One reason (of many) is that when the telescope was designed there was no way to know if we'd have capability to send anyone (or anything) to service it. Even

One reason (of many) is that when the telescope was designed there was no way to know if we'd have capability to send anyone (or anything) to service it. Even today we don't have a proven platform that could pull off a manned mission to L2, let alone a mission that would then fix an insanely complex telescope in-situ. (There are a few that could try, but no one's done it.)