There's a bug report for httpd dating back to 2011[0]. The nginx mailling list also has a grumpy person contemporary with that[1].
My guess is someone thought "httpd is a server running somewhere without a monitor attached, why on earth would it get a SIGWINCH!? surely it's available to use for something completely different", not considering users running it in the foreground during development. Nginx probably followed suit for convention, but that's pure speculation on my part.
Also that was before docker really took off (I'm not sure if it was around in 2011 yet; still in it's infancy maybe). Running it in the foreground didn't happen as much yet. People were still using wamp or installing it via apt and restarting via sudo.
They use SIGWINCH for gracefully shutting down workers but not the main process [0]. SIGQUIT is used for a graceful shutdown and SIGTERM for a sort of graceful shutdown (with timeouts).
SIGWINCH is apparently used for an online upgrade [1]. Because it only shuts the workers down you can quickly transition back to the old binary and old configuration if there's a problem, even after upgrading the binary or config stored on the hard drive.
I'm sure there are other ways to get a similar capability, but this set of signals is apparently what they came up with.
