the avengers - Why has the role of Bruce Banner been played by different actors in the three films that Hulk has appeared in?

The Eric Bana version of the Hulk was critically panned, receiving poor reviews from critics and fans alike, thus, Marvel called for a reboot to be tied into their Avengers plan.

From what I understood, Edward Norton quit after the 2008 version due to a lot of internal disputes (more information can be found on this answer to a similar question I asked before)

Thus, with the departure of Edward Norton from the Avengers Universe Hulk, Marvel hired Mark Ruffalo to handle the role, and from <Rumors> rumors I have found, he will continue playing the Hulk in a planned release, due after the Avengers 2 </Rumors>