Thanks to the recent IGN article, we have a better idea of why they changed the game from their original vision:
According to the game’s visionary, Brian Allgeier, Overstrike began when “a lot of people were itching to work on more cooperative multiplayer games. Even though we’ve done a ton of multiplayer modes in the past for Resistance and the Ratchet series, this was an opportunity for us to really delve deeply into a co-op experience campaign.” And the spirit of Fuse remains the same as Overstrike’s, even though other parts of the game have since changed completely.
“The game started out with a much more stylized and campy direction. We were actually going for something on the level of Ratchet & Clank, except with humans,” Allgeier said. “Maybe it was going to appeal to gamers who, we thought at the time, might be in their late teens. The industry’s changed quite a bit… We would focus test the game in front of a lot of gamers, and get their opinion. These are people that regularly play PlayStation 3 and Xbox games. We started to discover that everyone thought this was a game for their younger brother. We would hear this from 12-year-olds. So we decided that we needed to make a game that had an older appeal.”
Allgeier later said that “that’s kind of the reason for the delay. It’s like moving the herd of elephants, or turning the ship. Just turning a massive ship into a different direction and charting a new course.”
Naturally, the simple act of working on Xbox 360 has created technical challenges for the likes of Al Hastings to overcome, since as much as a guru as he is, he’d never worked on anything but PlayStation since graduating from Princeton. “Honestly, I feel like having learned the 360 has helped me put the PS3 in perspective. I see [PS3] now like everyone else saw it from the beginning. A simultaneously very cool and very frustrating machine. It has an immense amount of power, it’s just immensely hard to unlock.”
Actually, if I'm remembering my timing right as well as the context of this portion of the interview, Brian is referring to the style PRIOR to the E3 Overstike trailer, that style was never shown.