I installed a Kenwood DAB unit in my car five or six years ago, which has a CD slot, along with a front USB slot.
It’s never been used. I cannot see the point in cluttering up the car with a bunch of discs that only hold around 70 minutes of music, when I can connect my phone, my iPods, or a memory stick with gigabytes of music on using the USB port.
Many recent cars have a USB port for plugging devices into, along with a 3.5mm input for an analogue connector.
The dropping of CD slots has been going on for a number of years, you might as well ask why they don’t still have a cassette slot as well, seeing as how da yoof are getting all moist over cassette releases of new albums.