When Gohan finally returned to Earth with his full power unlocked to fight Super Buu, Dragon Ball Z fans were thrilled. Surely, after watching him focus on studying, fighting common criminals in a superhero costume and struggling against the likes of Dabura, this was the moment for Gohan to finally re-establish himself as the world's strongest warrior. Just as he ascended to Super Saiyan 2 against Perfect Cell, now Gohan was (as dubbed by fans) "Mystic" Gohan, who would tear into Buu and save the universe once again.
Except it didn't work out that way. Gohan did indeed wipe the floor with Super Buu in their first battle, but then Buu purposely self-destructed and regenerated some distance away to form a new plan. What transpired after that was a repeat of a common mistake throughout both the Buu Saga and DBZ as a whole: the failure to finish off the villain when a golden opportunity arises.
After finding Gohan and the others, Buu successfully taunts Goten and Trunks into using fusion to become SSJ3 Gotenks once again. Gohan initially tells the boys to stay back and let him handle the fight, while Piccolo senses some kind of deception but isn't sure exactly what it is. However, boys being boys, they couldn't resist the chance to fight Buu again -- especially knowing they had the added insurance of Gohan backing them up -- so Gohan stands back intending to watch, only intervening if necessary. And of course, Buu absorbs Gotenks and Piccolo, gaining more strength and wisdom than he'd ever had before, overwhelming even Gohan's newly awakened powers. But the real question is: why did Gohan let Goten and Trunks fuse in the first place?
Was it a touch of arrogance? After all, seven years ago Gohan made the exact same mistake against Perfect Cell. He knew he had far more power than Cell and wanted to drag out the fight to make the villain suffer a little longer for all people he'd killed. Ultimately, that overconfidence led to Goku sacrificing his life to save the Earth from Cell's last-ditch self-destruction. Against Buu, Gohan again knew he held the upper hand but agreed to cede the fight to Gotenks. Did he want Buu to suffer for a while as well? Was he just curious to see how Gotenks fought, or did he just want to see the fusion itself? Whatever the reason, it's pretty much certain that Gohan had a fair bit of hubris going.
Considering Buu had already murdered the entire planet's population -- including his own mother and his girlfriend -- letting Goten and Trunks take the lead for any reason at all was an astonishing lapse in judgment. He should've put his foot down and told everyone else to leave the scene immediately. If the boys refused, he could've taken a page out of Vegeta's book and knocked them out for their own safety. However, Vegeta's level of forcefulness is not in Gohan's nature, especially towards those he loves. He's even a bit of a pushover at times, and that allowed Buu to take advantage and absorb the power he needed.
That said, Gohan isn't entirely to blame for Super Buu's plan coming to fruition. The other culprit was watching the entire thing play out from another planet: Supreme Kai. Having fought Majin Buu millions of years ago when there were multiple Supreme Kais, the current Supreme Kai was already aware that Buu had the ability to absorb others into his being. All that time Gohan was training on the Kai planet, specifically to return to Earth and fight Buu, yet Supreme Kai never warned him about the absorption technique. Had Gohan known this vital information, he surely wouldn't have to give Goten and Trunks the go-ahead to fight Buu.
There is no one specific reason why Gohan failed to finish Buu off. It was a combination of misjudgments and a bit of overconfidence. But as happened with Cell, it reestablished the heroes' fatal mistake of failing to take every possible precaution and forgetting that the battle is never truly over until the villain is destroyed once and for all.
