The rule was brought in after the Newcastle takeover (rightly so). I can’t remember which way City voted, but they have obviously decided it doesn’t work for them and are challenging the nature and integrity of the system.
I imagine at the time they were content because they were winning and there would be less competition from Newcastle. Now they have some positions to fill and strengthening to do and probably want the very best available asap. They want to dominate the league, and to do so they have to maintain the levels they have now which takes money.
There’s also the added distraction to the premier league from the 115 case, which clearly plays a major part. The bubble could well burst after the result of that case and they will do everything they can to stop it.
Also, they have clearly realised that even winning a treble and four in a row they are still nowhere near as relevant as they thought they would be. Everyone admires the football and the achievement but still no one really cares about City in general. It probably gets under their skin a bit.
They will never be idolised for lifting a CL in the way Real Madrid are and the only way they ever will is if they keep winning and winning, which is only possible by being the most attractive club on a long-term basis which is where the dollar comes in.
Finally, they want to destroy the premier league ‘cartel’ by simply creating another one, that comes from money and power rather than accolades and prestige. Essentially they have gone nuclear.