How is it that these receivers can't get separation when Cassel and Weeden are the quarterbacks but can when Romo is quarterback?
I don't think it's an issue of the receivers not getting separation.
This offense, like most NFL offenses, are based on timing patterns. You may not get super separation, but you can get a step on a defender. And if that's the case, your quarterback has to get you the ball.
IMO, Cassel and Weeden lack the confidence to fit balls in to tight windows because they don't want to make mistakes. They don't have the overall awareness to scan the field, look off receivers and find guys open. And they don't have the talent Romo has to be able to successfully put the ball where it needs to be so only your man can catch it.
Oh, and they don't have the experience with these receivers Tony has.
Good/Great quarterbacks make average receivers look good and good receivers look great. For goodness sake, look at what Brady has done his entire career. Look at what P. Manning has done all his career.
Tony Romo has made the likes of Patrick Crayton, TWilliams, Cole Beasley and even Dez Bryant (though Dez is a superstar, he's not exactly the best route runner) look better than they are.
Cassel and Weeden are going to make them look average because they're below average in their ability to read defenses, process information quickly and get the ball to the open man in a spot where only their receivers can catch the ball.
The NFL is a game of inches. Tony Romo only needs inches. Cassel and Weeden need yards between the receiver and the defender for them to complete a pass.
And, that, my friend, is the difference.