Because it’s time.
Background: this Mueller guy is ostensibly digging into Russian interference in the US election, but a guy named Nunes is actually finding stuff out about that. Meanwhile, Mueller has indicted Manafort, ostensibly for something vaguely related to Russian interference in the US election - but that’s quite a reach, to be honest.
Why the indictment? Because it’s late October. That’s the only explanation that makes any sense. Why does that matter? Here’s why:
It would have had much more impact in late October 2018, or even 2020, but things were going so badly he probably figured he had to use what little he had before it went stale in him.
And what are Mueller’s motives? Unclear, but he has enough connections that many called for him to recuse himself.
Yes, this sort of phony indictment happens a lot. It’s called an October Surprise.
Come to think of it, it could also be intended as a counter to what Devin Nunes has been doing to public opinion. Either way, it’s all about manipulating public opinion, and thus election results.
(What, you thought it was about something else?)