I can guess what's happened over the last few years. The cartel clubs were hanging their hats on us getting a ban from UEFA, at which point the PL would also presumably have charged us. The PL claimed to be investigating in 2018 but probably did fuck all of any note. Once the UEFA case was chucked out by CAS, Plan B went into operation, which involved the PL starting to dig around.
They could only go on the stuff that Der Spiegel published but the Etihad stuff was probably off the table. All they really had was the time-barred stuff, Mancini's contract and Fordham, which UEFA knew about a few years earlier but having spoken to us weren't seemingly too bothered about.
So they've got some bits and pieces, but they're pretty weak and I suspect the PL legal advisors have told them this. But Masters has a problem, as the cartel clubs are on his back. For a while it looks like Arsenal are going to win the league and it looks less obvious than bringing charges when we're ahead. So after we lose to Spurs, urged on by Levy on behalf of the cartel clubs, they insist on sticking the boot in. Masters, like Ceferin before him, really doesn't want to do this but he's under pressure. So his way out is to agree to bring charges he knows have little or no hope of success.