When Enterprise was new, it seemed like they did a lot to distance it from the rest of Star Trek. Maybe they were trying to attract non-trekkies or something? Whatever the case, they didn't advertise it as "Star Trek: Enterprise". Just "Enterprise" (until later seasons.) The commercials/teasers left it easy to wonder if it was Star Trek or not, at least for people who didn't watch it. And the credits music is just another example of a departure from the rest of Trek.