The short version is: times have changed. Technology has changed. And most importantly, how people watch television has changed. Many people have moved on from cable television to on-demand streaming services.
For those who continue to watch "linear television" (cable, satellite, or OTA TV and similar functioning streaming services like YouTubeTV), they often find the DVR offered by the service provider to be "good enough" for the limited amount of television they're actually recording and watching later. As good as TiVo? No, probably not, but "good enough" often wins.
It's simply not worth the investment for service providers to continue to cater to the absolutely minuscule number of remaining TiVo users. (It never was, if we must be honest.)