Frankly, I don't know why ... it's wasn't a bad movie ... it didn't have those "would they get on with it?" dragging bits, it didn't have those "I'm embarrassed to be watching this" bits, and a lot of it was "cute" and "fun". The cast worked well enough, with the "adult" half being right on target (Tim Allen, Courteney Cox, Chevy Chase, and Rip Torn), and the "kids" half being serviceable (the two girls were great, the two guys, eh). Heck, it even had a "surprise element" (which would be a huge "spoiler", so I'm not going to detail it), that I didn't see coming at all, yet made seamless sense within the flow of the storyline (this does not happen to me a lot).
What I think folks who do not like Zoom are missing is that it is a parody, not just a "kiddie flick" ... shot after shot (from the opening flip-through-the-comic sequence to the UFO "bouncing" across a desert landscape) are cribbed from recent Marvel movie adaptations, but these are subtle (like having a UFO on it's edge standing in for the Hulk, rather than some character), and generally easy to miss (tell me that Summer/Wonder's "using her powers" shot towards the end isn't a copy of JeanGrey/Phoenix from X-Men3, right down to the hair movement). Unlike many of these films, the homages here are (with the one exception of where the theme from The Good, The Bad, and The Ugly figures prominently) woven into the context of the story, and only hit you later.
The story, however, is a little thin ... the title somewhat oddly refers to "Captain Zoom", a superhero (Tim Allen) who had been the leader of a "teen superhero" squad, but had lost his (super speed) powers in a climactic battle with his brother ("Concussion") who had turned bad (you could tell from the glowing red irises) by the Military trying to boost his powers with a big Gamma Ray generator (Stan Lee, your lawyers are on line one). Concussion destroys the original team, falls into some trans-dimensional vortex, and disappears for 30 years. Somehow he's coming back, and blames Zoom for his long isolation. The Military discovers that he's on his way back, and collects the now (nearly) powerless Zoom to train a new team, without telling him why. Yadda, yadda, yadda ... the good guys win, while thwarting the Military, and saving Concussion. This last bit was (remarkably) the only gaping hole in the plot ... as Tim Allen's character does exactly what the scientists say he could do (if he only had his powers), but in meetings that he is most pointedly not privy to ... I guess it was a good guess on his part.
Anyway, Zoom is a fun movie, especially if you're looking for some PG fare. By the way ... the character of the 6-year-old girl is hilarious, if you've got young kids they'll probably love this just for her!