As someone who used to hang around TrekBBS back in the day, there are actually many conservative and libertarian Star Trek fans.
It always baffled me also, but I think many of them were/are TOS fans. Kirk's swashbuckling, individualistic, break-the-rules, throw-a-roundhouse-when-you-need-to style disguised Roddenberry's socialist utopia that existed in the more civilised parts of the Federation. Certainly more so than adventures of the tea-sipping, conference-chairing, "I think I'll surrender in my very first appearance" Frenchman who followed him.