There are a few separate issues here.

1. There is a style issue. Stem shifters are better than downtube shifters if you want to ride in an upright position; therefore wannabe racers and roadie snobs (yes, including myself, 30 years ago) tended to look down at them.

2. There is a price point issue. Stem shifters (along with cottered cranks, extensions on the brake levers, bolt-on axles, steel rims, and so on) tend to be (for lack of a better term) symptomatic of a cheap bikes. Even so, you may find them on very fine bicycles, including Schwinn Paramounts.

3. There is a mechanical issue. Comparing stem shifters with downtube shifters, downtube shifters are the simpler and more precise mechanism because there is less cable housing and fewer bends in the cable, therefore a more direct connection to the derailleurs. This is true, of course, but bogus since by the same logic, we would prefer stem shifters over bar end shifters, or brifters; and we don't.

4. On the other hand, there is also a size issue. Downtube shifters may be within easy reach on bicycles with small frames, but a tall rider on a tall framed bicycle has to reach much farther down. When I upgraded from stem shifters to downtube shifters, I really missed them!

5. Finally, there is the Campagnolo issue. Campagnolo never made stem shifters; therefore stem shifters must be bad.