Isn't vinyl the most popular physical medium by sales right now? Sure, 78s aren't as supported, but the first Technics turntable I clicked on can play them, and the Wirecutter has a recommendation that plays them. "Obsolete" is a stretch.
Basically the technical reason is that the 33/45 record groove is read laterally, while the 78 groove is read vertically. So both cannot be properly achieved with the same reading apparatus.
Modern records use a so-called "microgroove", usually crystal needles in the cassette, and one or so grams of tracking force. You probably wouldn't pick anything up off a shellac record, if you tried playing it.
Just make sure you invert the equalizer / preemphasis before you change the speed.