Researchers put Shellshock proof of concept exploit code into fuzzers[0] and managed to find five more bugs subsequently[1].
Heartbleed required a patch to OpenSSL, which is typically a dynamic library. I'd wager that without publicly available exploit code to test with many admins would have just installed the patch and failed to restart the server processes, leaving their systems vulnerable.
The solution to improving patch deployment timelines is automated updates, not having admins always online. Admittedly this is still an area that is underdeveloped.
[0] http://lcamtuf.blogspot.com/2014/10/bash-bug-how-we-finally-...
[1] https://en.wikipedia.org/wiki/Shellshock_%28software_bug%29