To the contrary, they struck when the fire was hot.
For whatever reason, the Steelers "blinked" on Whisenhunt (which had to make him a bit antsy about whether, if he waited too long, he'd be hired by anyone). The Cardinals jumped in and pulled the trigger.
We don't know the back room details, but there's some MJ rumor/innuendo that Whisenhunt was orginally a little reticent about coming here in comparison to a possible Steeler gig but that his agent made him take a more realistic look at the Steeler situation. Once Ken bought into coming to the Cardinals, he became more and more enthusiastic (AND prepared for his second interview).
I'm guessing that the Cardinals rated Whisenhunt higher than Cameron, but was afraid to put their eggs in one basket and risk winding up with nothing should Ken accept a Steeler deal.
Once Whisenhunt expressed enthusiasm about coming to the Valley, there was no longer any reason for the Cardinals to consider Cameron.