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The end of the Dark Knight leaves
murders needing to be covered up in order to maintain the good that Harvey Dent was able to do. Batman decides that he needs to be the one to take the fall in order to protect Gotham.
Why does Batman need to be the one? By this point, the Joker is in custody and has been known (both to viewers and the police) to use henchmen. No one would believe the Joker if he was to deny it, so why couldn't everything have been pinned on the Joker or any of his henchmen?
It's been a while since I watched The Dark Knight, but here are my thoughts based on what I recall:
It keeps the criminals in prison. Harvey was as much a victim of the Joker as anyone else (especially in what drove him to be Two-Face), but he still committed the crimes he committed. If that were to be found out, all of his cases would be called into question, and everyone he helped put away would likely be released (which would be very bad for Gotham).
It allows Gotham to stop relying on Batman. By using Batman as the scapegoat, the people of Gotham would turn to their other heroes (Dent's legacy, the police force, etc), thereby releasing Bruce from the burden of Batman. Batman disappears, and it never has to come up that Harvey committed those crimes, because someone else has taken the blame, and that person is nowhere to be found.
Dent's death doesn't match Joker's MO. The Joker likes gore. He likes explosions. He likes fear. He likes spectacle. None of that happened with Harvey's death. If the Joker would have gone after Harvey, he'd have taken a city block in the process (just like he had already). At the very least, he would have done something like throw him off Wayne tower and ensured that he hit the street below, where people would have witnessed it. That leaves the reasoning: if Batman could kill Gotham's White Knight, who's to say that he didn't kill the others (those that Dent did kill), as well? So, if Batman takes the fall for Dent's death, then the logical thing to do is take the fall for the murders that Joker doesn't claim. An astute investigator would easily see the issue with blaming Dent's death on Joker and start asking questions.
IMDB has a good list of reasons, as well, some of which I've also come up with and wrote here.
The given answer is perfectly acceptable, albeit a little weak on the 3rd point as has been pointed out, but as an addendum I'd add:
Blaming the Joker would be corruption; the very thing Batman and Gordon have pledged themselves to rid Gotham of. It's still a form of corruption for Batman to falsely assume responsibility for Dent's crimes, but as he is assuming this responsibility willingly it negates the destructive impact to an unwilling party.
If Gordon and Batman were to re-assign the actions of Dent to the Joker, they would be shifting blame onto someone who is innocent of those crimes; although guilty of much greater ones. It's the fundamental act of corruption that they have decided to destroy.
As the Feudal Lord of Gotham, its Lord (and secretly its Knight), Wayne shoulders the burden of exile for what turns out to be both of his personas.
Of course, we see the damage of this in The Dark Knight Rises, when Bane realises this very point: Corruption is Corruption, and both Gordon and Batman are just as guilty of it.
When he reads Gordon's confession, he weaponises this information to undermine the legacy of Gordon whilst venerating the reviled memory of Batman, whom he believes is crippled and watching from afar.
Whilst ostensibly a device to free Blackgate's prisoners (and get a lot of sympathetic soldiers) the genius of this scene lies in its cruelty. Bane destroys Gordon, whilst raising Batman to the heavens as a savior... at the very moment he can do nothing to intervene.
It would be a bitter pill for Wayne to lie wrecked and ravaged, hearing his beloved subjects finally calling for their Knight once more, only for those cries to go unanswered... Brilliant writing, but requires a lot of through-line-following to see the genius of it.
