The issue is MV2 made it way too easy for malicious browser extensions to do bad things, like read the content of every page you visit. MV3 makes it much harder for malicious browser extensions to do these things, but makes it harder to do things like intercept network requests.

Then allow a savvy user to choose to keep MV2 mode via an opt-in control instead of depreciating years of hard work by non-malicious extension authors. uBlock Origin is, in fact, the ONLY browser extension I use in Chrome, as Firefox is my main browser.