Source: me. Was working at Adobe at the time, probably at the thing that actually led to the demise of Flash (not that it was my intention in any way to contribute to the death of Flash, the decision to kill it was a surprise to me as much as it was to anybody else).
Flash cost Adobe a lot to develop (it was a big team). At some point, I speculate[+] that Adobe simply decided they no longer need the runtime, since the browsers have/will soon have all the capabilities that they need, natively. So why waste the money?
[+] I don't know for sure (e.g. I didn't participate directly in the decision); but still, I believe my speculation is much more informed that the typical net user who just noticed the coincidence and wanted to believe the narrative.
I think that ignores a very very significant reason for the browsers getting to that point of being able to supplant Flash being that is was terrible and security nightmare and Apple had shifted mindshare against it and shown it wasn't needed in a mainstream platform. It wasn't just Adobe graciously terminating a successful platform that gave them a lot of influence.