Let me sum it up simply without even reading that.
Because Chrome has become the dominant and most compatible browser.
As with anything, Google became overly comfortable and started stripping features and adding telemetry.
Chromium which is open-source came to the rescue. Google didn't like that so they made syncing and similar functions closed and blocked workarounds.
Seeing that significant feature gone, the community started customizing it to their liking. One of the OGs and most popular being Bromite which ceased development and Cromite stepped in it's place. There are as eluded to above various other flavors including the proprietary Vivalium (or however it's spelled) to OP, Vivaldi, and many others.
MS Edge eventually went Chrome based among others.
Mozilla's CEO back when giving himself a pay boost among other mismanagement steps not directly his fault contributed to their fall in popularity and rising dominance. They have their own flavors like Fennec, Mull, Ice Raven and others for the same reasons, privacy and lack of options.
TLDR: Here we are today. Born from the lack of competitors, options and to add customization options.
Facebook
Twitter
Instagram