Well, to adjust the volume on physical keys, you just:
1) Press the key you want to adjust volume up or down
To adjust volume on Touch Bar keys, you have to:
1) Press the Touch Bar to turn it on in case it fell asleep
2) Press the volume key 🔉
3) Finally, you can adjust the volume
So assuming there's no bugs and the display isn't acting up, even simple adjustments take 2x to 3x as much effort compared to physical keys. It might not seem like a big deal, but after a while it kind of becomes a hassle. And if the Touch Bar did have bugs, a system restart is the only reliable way to fix it.
To be fair, the Touch Bar did have some nice features, especially with certain apps, but in the end, I think people just decided the potential upsides just weren't worth the hassle (or the bugs). Maybe if Apple stuck kept it for a few more generations, the issues could have been ironed out? Who knows. 🤷♂️