Why would silicone caulk not cure?
Expired caulk

You bet it expires, and fast.  That is because of the chemistry of this type of caulk.  The stuff is nearly magic, but that comes at a price.  

Extremes of temperature are not kind to it either.  That's not a surprise, even latex paint has that problem.  

I have had your experience myself, grabbing random caulk found around the shop and going "this must be OK", only to have to redo the project. Removing the old material completely is a huge chore, and the job will fail if you don't.  Never again!