Because there is absolutely no indication of Izuru existing within him after that. Even after going Super Saiyan, he still acts just like regular Hajime. I assumed they did merge but it never really gives any indication of that which is what bugs me. Instead of saying that they merged but Hajime's good side just won over the bad for dominance, it just goes on without explanation and makes it look like Hajime just trashed the Izuru side completely. Which goes against what was supposed to happen. There should have at least been a short segment where the two personalities explicitly merge and Hajime just chooses to live with it rather than how it was portrayed.
I think you're looking at it wrong. Hajime doesn't have a good side, it didn't "win out" as you say. The Hajime you see at the end is the Hajime who accepted responsibility for his actions. Someone who has the resolve to create a new future for himself and his classmates. It's distinct from his Kamukura self (who completely lacked Hope) and his pre-Hopes Peak academy self (who lacked Despair).
Anyway, a good story treats its audience with respect and assumes that they read between the lines. I don't think that everything needed to spelled out (as it would needlessly drag the ending).
This
Hajime Hinata: But still, I’ll keep on living.
Hajime Hinata: I’ll keep on living as Hajime Hinata
Hajime Hinata: ... My future is here.
Is the only thing matters.