Because time is linear and sequential, the reality and consequences of evil are permanent: it happened, it leaves a trace - changes the course of things.
Evil can be healed, but not reversed.
Because free will is real, some have not repented - and some perhaps may never repent.
Thus evil may be permanent - what then to do with it?
How can there be Heaven - how the New Jerusalem - when evil remains?
Evil must be sequestered - and that is what Hell is for: to sequester the unrepentant.
Hell is to sequester the evil, until they repent - or if they do not ever repent, if that is how they choose: then to sequester them for eternity.