The thing about "Smallville" is that it is supposed to be about Clark Kent's formulative years except in the way the series has been presented, they've exhausted that part and now are telling stories about his early adult life, while still holding strong to their old formulaic style and of course reinforcing with an iron fist "No Flights, No Tights".
The problem is that the series ran so much longer than they ever expected, so they've had to postpone his transformation into Superman unnaturally long while still coming up with new stories to tell that could move the saga forward. I assume the reason they didn't just turn it into a Superman series is that, like I said, the whole idea was to reinterpret the Superman myth for people who'd be put off by the cape and tights. That's the fanbase that made the show a success: the people who watch for Clark, not Superman. Yes, they're also trying to appeal to the comics-fan side of the fanbase more and more these days, but I can understand them not wanting to alienate the show's original fanbase in the process, not wanting to fundamentally change their brand identity.