User blog comment:KidVegeta/Canonicity/@comment-3000576-20101008233507/@comment-3000576-20101009011748

Here's a quote (note: It's a possibility, not a fact.:) There is also a possibility that the film takes place in one of the alternate timelines of the series. If this film takes place during a timeline where Future Trunks, and in turn King Cold and Frieza, does not confront the Z Fighters, then this might explain why Cooler is intent on nothing more than seeking vengeance for the shame caused by Frieza's defeat, and why he appears to be completely unaware of the deaths of his father and brother on Earth.

Another alternate timeline possibility is that Goku returns to Earth before Frieza, King Cold, and Future Trunks arrive, and did not practice becoming a Super Saiyan along the way from Namek; this could explain why Cooler was intent on seeking revenge on only Goku and why Goku had to become enraged to transform into a Super Saiyan.