USB flash drives took way longer to catch on than most people remember, thanks to how ubiquitous they are now. It took ages for them to become large enough to be worth a damn, for the plurality of computers to be compatible enough to support them, and for them to become affordable enough for anyone other than nerds or businessmen with an expense account to care. And then USB 2.0 just would not gain widespread adoption for what felt like about a century, so even what was available was inevitably agonizingly slow even if it had any kind of capacity.

There was a solid chunk of time between about 1997 and 2006 when a CD-R was not only monumentally cheaper than flash media but was also much more likely to work in any random computer or other device you stuck it into. Prior to about 2003 you couldn't realistically even buy a flash drive that held as much data as a humble CD-R in the first place. In 2004 a 256 megabyte USB flash drive would run you $50 and operate at piddling USB 1.1 speed, but a 700 megabyte CD-R was 20 cents. That helped the CD-R and certainly the DVD+/-R formats to hang on well past their supposed sell-by date.

(And I just checked, since I was morbidly curious. A Verbatim CD-R still costs about 21 cents per disc at Microcenter. Yes, you can still buy them.)

A later large portion of the application for writable CD's was, I'm sure you'll remember, good old fashioned wholesome piracy. At 20 cents each it was cheap and easy to run off a copied CD full of whatever to give to your friends and not expect to get it back. So even after flash drives became affordable, they were never never affordable enough for most people to do that.