The Incredible Hulk wasn't an origin film. They did a flashback to his origins in the opening credits, and then moved to him being established as The Hulk and being hunted by General Ross.
It was referred to as a "reimagining", a sort of hybrid of a sequel and reboot. The key here is sequel, as it is supposed to take place after the events of Hulk's origins.
This article from around release time mentions this concept, as did many others in that day.
Not a sequel, no no. Not a remake either. Not even what’s recently been coined a reimagining. They're calling The Incredible Hulk a reboot. After Ang Lee’s brave, interesting but deeply flawed 2003 Hulk failed to engage either audiences or box office tills, the future of the big green smashing machine seemed in serious jeopardy. But Marvel and Universal are back for a second crack with a new director and an all new cast, and hopes are high.
As this quote touches on, HULK was a critical failure, yet the character is still very popular and they hoped to make more money from him. The Incredible Hulk was also part of Marvel's "Phase One" of the Marvel Cinematic Universe, establishing the character for his role in The Avengers. It's also important to note that, as far as movies go, Hulk is Universal's property, and Marvel Studios was not involved with the first film.
Reboots are not necessarily uncommon in short timespans these days. Of note is Spider-man, which saw The Amazing Spider-man just 5 years after Spider-man 3. And Spider-man's character is being rebooted again after only two years, making an appearance in Captain America: Civil War, and then in a new film sometime in the future.
As far as financials go, Hulk has made $245 million worldwide, on a $137 million budget. That's not the best, but it's not the worst. The Incredible Hulk has actually done about the same, at $263 million on $150 million budget. But these aren't the $500 million, $1 billion blockbusters we're seeing now. This is likely why they originally weren't planning on another. However, with the popularity of Ruffalo's Hulk, they may now be considering the idea.