Because it is maintained by a for profit company and because I believe it defaults to sending back telemetry data to said company, though you can opt out of that. Those are the reasons I'm aware of anyway.
Desktop Linux is becoming more mature so there is less need for an "easy" distro. Also, Canonical (company behind Ubuntu) has been pushing their tech (Mir, snaps) instead of contributing to really open alternatives that everyone else uses.
They had a lot of missteps over the years (e.g. at one point, they shipped with Amazon ads in the OS). Currently it's the way they're pushing Snap (which is a lot like Flatpak, but proprietary and only really used by Canonical (because it's proprietary)).
As someone who has been down the rabbit hole, I was running Gentoo with linux-libre with my use flags all set up to install only what my machine and set up needed. This is the correct answer.
I’ve been back on Kubuntu for about 8 years because it works for me.
It's a Linux flavor used by novices, it's straightforward to install and requires very little configuration to be usable as a document editing workstation.
It's used often by novices, because outdated articles keep telling them it's "the best Linux distro". Canonical has gotten very corporate over the last several years, forcing things like snap onto users. Ubuntu used to be the number one user friendly distro, now they shove ads in the terminal. It's not getting hate for being easy to use, it's getting hate for marketing itself as such, then forcing corporate bs on the user (who are often new to Linux). Many other user-friendly distros have not seen the same amount of hate, because they aren't objectively bad.
Oh man, is this true? Last time I daily drove Ubuntu was like 10 years ago. Ubuntu stood for the "it just works" of Linux. That's so sad they've fallen so far.