Because ceramic-disk technology is more expensive than the ball valves and springs that Delta has used since dirt was new. :-)

Ceramic cartridges/disks are very tough and (typically) won't wear down like other materials. My $.02 (and I'm putting my money where my mouth is with non-ceramic Moen and Delta faucets throughout my house) is that there are plenty of good, high-quality, long-lasting faucets out there which do not use ceramic, so it is not necessary to avoid faucets without ceramic disks.

I had them in a previous home and they did work smoothly and the units had a nice heavy feel to them..........but.........unless the internal design has changed over the years I wouldn't have them again.

One year we had problems with the HW tank where something disintegrated and the small bits of ceramic from the HW heater flushed thru the system and lodged in every faucet/shower ceramic disc thruout the house. Every week for several weeks I had to take the units apart and clean them..........what a pain!
What's interesting is that my inexpensive faucet in a hobby room never had the problem..........only the expensive faucets!