It was a brief period where many people wanted a feature (usually playing CDs) that was not available on the installed unit. You may have also noticed the market for new aftermarket car stereos suddenly rose, and then collapsed.

These days, it's all about adapters.

Many aftermarket radios still support that detachable face feature!

The stock radio in my 1999 model year car had a couple buttons that were removable (instead of the entire face, but it disabled the radio just the same). But no one was stealing stock Ford radios, as far as I know.

Climate controls, navigation, and other vehicle settings are now in the central touchscreen display. This makes the radio more of a command center, a 3rd party stereo would not be able to integrate with all the functions.
Removable panels were once a deterrent against theft, modern vehicle security systems and the overall integration of the stereo into the car's electronics have made them less necessary.