History's gotta count for something; the fact that the Earth One Dick Grayson was the first Robin - and remained the only Robin for something like forty solid years - certainly makes him the definitive Robin in many people's eyes. But it's not just that Dick was the first out of the gate; he also set the template. The grinning kid who leapt first and asked questions later, who provided a thrill loving contrast to Batman's grim drill sergeant, who brought out an unexpected paternal side of Bruce Wayne's character - all these elements would be developed (Tim Drake), deconstructed (Damian Wayne) or inverted (Jason Todd) by later Robins, but the paradigm begins here. When the public at large imagines Robin in their mind's eye, it's most likely this Dick Grayson Robin they're seeing. (The fact that he's easily the most well represented Robin in mass media probably doesn't hurt.) It also doesn't hurt that Dick Grayson is a legitimately terrific character in his own right (particularly when he became Nightwing). He has one of the best origin stories in superhero comics, almost as primal as Batman's: a pair of trapeze artist parents sent plunging to their deaths by protection racket gangsters. At worst Grayson could be annoying and whiny (cue the "Robin, the boy hostage" jokes); but by and large, his tenure as Robin was characterized by tenacity, bravery and ingenuity. Introducing kid sidekicks into superhero comics was in most respects a pretty dopey bid to give kids "someone to identify with" (as if kids didn't pick up a Batman comic to, y'know, read about Batman), but the pandering was forgivable with Dick Grayson because he never talked/played down to his readership. Robin was a clever, capable kid who got to chum around with Batman. What could be cooler?
C.B. Jacobson pops up at What Culture every once in a while, and almost without fail manages to embarrass the site with his clumsy writing. When he's not here, he's making movies, or writing about them at http://buddypuddle.blogspot.com.