Why does Jurassic Park succeed where the sequels fail? Two words: Steven Spielberg. (Yes, I am aware he directed The Lost World too.)
Spielberg is a master at making an action/thriller with emotional depth. Not only are the dinosaurs cool yet terrifying, but you care about the people being terrorized. I dare you to find someone who does not fear for Tim and Lex, screaming for help, as the T-Rex destroys their car.
While we're on it, the T-Rex attack is one of the finest scenes in movie history. No music, just the roar of the T Rex and the sounds of destruction. Amazing.