Well... he kinda did, it just took him a little while after his ressurection.
It's easy to miss considering how much of the last book focuses on Harry going cross country, but after his resurrection he spends rebuilding all his empire in secret while wassisname the Minister ran the Ministry into the ground out of terror. Then when everyone knows he's about he gives them a year to panic while he has his most powerful and dangerous enemy assassinated. As soon as that's done in like a month or two or he takes control of the Ministry and Hogwarts without a fuss.
I mean, seriously, it's suggested last time it was out and out war with Voldemort. This time the Order of the Phoenix is literally the only piece of Britain putting up any resistance.
If it weren't for the fact Harry was still alive and kicking, Voldemort had essentially won in a tiny amount of time.