Contrary to popular myth, Marino DID move around, it just wasn't much. He moved just enough to avoid the pass-rusher but was able to do so while still keeping his eyes downfield. He was NOT a scrambler, but he had excellent footwork and moved as much as he needed to.
As others have said, Marino had a VERY quick release but he also had incredible pocket presence and awareness of the pass rush.
Bledsoe, and others like him, tend to focus on the rusher and not keep their eyes downfield or else they look downfield and are completely unaware of the pass rush.
It is a VERY difficult trick to master (I know, I've tried and failed for the most part). You have to keep your eyes downfield, absorbing everything in your view without focusing on anything close to you. It allows you to see the pass rush, the receivers in their patterns, your blockers, even the game clock, without completely focusing on any of them. Being able to take in all of that input and make decisions on it is the difference between a great QB and a guy with talent.
Marino was the best I've ever seen at this, Bledsoe is one of the worst, at least of the better QBs, Chad Hutchinson was probably the worst I've ever seen. Bernie Kosar was another guy who was pretty good at it and needed to be since he was as much of a statue as Bledsoe but he had a better feel for the rush the Drew does.