Because City's IT security (used to at least) be a comedy show. They didn't want any more press attention on it than necessary
There's this thing in security called "ethical disclosure". Basically, if you as a security nerd find a way into the system of a big company then you report it to them and they'll give you a "bounty". Some of these are very generous and in the tens of thousands for ultra severe ones. Others not so much.
About 15ish years ago, I "ethically disclosed" to City that I could watch webcams of some of their internal meetings. Literally there were webcams in a bunch of meeting rooms that you could just sit and watch if you fancied it, without credentials, because they weren't properly locked down. They didn't even reply to my email let alone say cheers let alone say "oh you've saved us a bunch of money there, here's few quid". They fixed it within a day or two though. So if I bothered to go on a fishing expedition and find a bunch of other shit then what's my incentive to disclose if I wasn't a City fan? This is the stuff that leads to the email situation.
Do you know this "hack" that people talk about with Liverpool? Do you know what it was? When some staff left City, City didn't bother to cancel their access to our scouting apps. So they booted up their laptop one day, went to a webpage that hosts these types of things and were automatically logged in as City employees. Then probably pissed themselves laughing as they fed information to their new bosses.
The fact that our email system was hacked is possibly the least surprising thing that has ever happened in the history of the world. Our website is hackable. You can access City+ without subscription if you're technically inclined and know how to do a bit of JavaScript editing. Our ticketing system was hackable for a long time through credential spoofing. Football as an industry is laughably insecure compared to other industries. I absolutely guarantee you right now that there are people out there who are reading the emails of every major CEO in football. It's not a serious industry when it comes to infosec. Especially outside the PL, their security is done by some guy's kid who they think knows "a lot about those computers" because they play Roblox or something.
If a properly motivated hacking group ever decided to target ransomware towards the football industry then everyone would be absolutely fucked or they'd get away with millions.