I figure that, since Shimano's brifters not only aren't rebuildable, but also that they expire just from sitting in the garage, that there is a relative scarcity of Shimano brifters in the marketplace versus Shimano rear derailers.

Whereas, with Campy, the Ergolevers on all of the seldom-used bikes that eventually get sold, with some of these bikes being parted out as "parts available" on the market, are still serviceable years longer than would be Shimano's brifters.

Also, in the past, with Campagnolo having used wider cassettes and thus a closer sprocket-to-spokes distance on their rear hubs, many a Campag rear derailer met it's maker after being shifted into the spokes.
This has resulted in a relative scarcity of Campag rear derailers versus Campag Ergolevers available.

Lastly, with Shimano offering MTB derailers, many of these get substituted for a bike's original Road derailer, leaving a few more Shimano road rear derailers looking for a home.