They established that rule at the very beginning of the series, wanting to differentiate it from other Superman shows that had come before and to concentrate on Clark's humanity. But come on now... that was over seven years ago and the show has come a very long way since then.
Here is Gough's response to a question regarding whether they would be sticking with their original rule or whether Clark will learn to fly this season:
"Yes, we are sticking with that policy as far as Clark is concerned. (Obviously Bizarro can fly as well as Kara.) But following up on what Chloe said about his lack of flying in the season premiere ("Clark, you need to get on that"), Kara will be giving him 'flying lessons' later in the season. Although with Clark, they end up being more like 'falling lessons.'"
Especially in view of the fact that Kara and Bizarro can fly, and the fact that when Clark was possessed by (was it Jor-El?) he was able to physically fly as well, this just seems plain stupid to me. That last item shows that it's not because that power hasn't "matured" yet.
I'm sure that at the start they believed that the show would last four or five seasons, tops. But here we are in season 7 with a probable 8th on the way. Fans have been clamoring for this for ages, and it's not like some "will they or won't they" sexual tension thing that would kill the show once it happens (anyone remember the old Bruce Willis series Moonlighting?).
Continuing to deny Clark the ability to fly is going to start looking really stupid very soon in light of all the other characters on the show that have the ability.
On another note, I found it really annoying in the episode "Kara" when she referred to the El family as "dysfunctional." Gough reiterated this in the interview:
"Kara will give him an insight into his family (which is seriously screwed up) ..."
I realize they've played fast and loose with the Superman mythos but to me this is really pushing it. Clark's Kryptonian family has always been portrayed as noble, wise and intelligent. But now? Now they want it to be closer to Everybody Loves Raymond?
I've been a big supporter of this show, but I predict that this is the season in which Smallville truly "jumps the shark." I had previously thought that this (#7) should be the final season, but now I'm thinking that perhaps they should have wrapped it up in season six. At this rate season eight will descend into pure parody.
Source: TVGuide.com via Comics2Film