"Probably because the point at which blank CDs and burners became affordable for most people (around the tail end of the '90s) was also the same time that MP3 was exploding in popularity."
Likewise, I don't think I ever burned any actual audio CD. I bought a portable CD player that also played MP3 files of discs and a wired cassette adaptor to use it in the car. When that broke, I got one of those cheap "fat" USB stick type MP3 players which used the same cassette adaptor. Next car had a 3.5mm AUX socket so now the adaptor went in the bin. Next car played MP3s off the built-in CD player. Every car since has had a USB port for playing MP3s direct from a pen drive. Evolution in action :-)
(NB, all company cars, so I didn't get to choose the in car HiFi systems.)