I can think of a couple of reasons...one being "Price"...
You can run one of the old "CB" tube amps and get a clean signal out, without splattering 40-up and 40-down if you have the amp and radio properly tuned...
With my Cobra 2000 I run either a "Kicker 500" (1x3 tube) or my Palomar 300A (2x4 tube), and have a very clean sounding signal both on AM and SSB...Don't bleed on anything in the house or my neighbors house's...and get complements daily on how good and clean the radio sounds...
Tubes ar fairly expencive now days tho, but they last forever as long as you don't push the snot out of them...