inconsistent (client and server) software support
This is taken care of by preferences. Set the highest you are able to support and keep pushing it higher. Clients do the same. It's negotiable preferences, no flag days, everyone wins.
At a guess, I'd say a mix of laziness, and worries about additional CPU overhead.
These are more common.
I had to pull Apache 2.4 out of Sid
Unfortunately port/package repos can be a bit behind state of the art. Locally... untar ; ./configure ; make is not that hard to learn.
corporate policy
If only as to doing mandated things like TLS termination and DPI.
Turning off non-FS algos breaks SSL for a *lot* of people.
Set preferences, not hard cutoffs.
So the many servers where OpenSSL isn't getting upgraded any time soon can't do it either.
I've only found compiling new software on old systems to be a problem like this. ie: 1.0.1 won't compile on them. I grant that it can be hard to migrate off old platforms.
submit bugs against the web server packages from the usual suspects (debian et al) asking them to turn on forward secrecy by default?
Legitimately squeaky wheels get greased first.