Personally I think it's just become so common knowledge that tanks and healers receive the most coms, that people give them their coms based on just that fact. Some don't even take notice how good or bad a dps is doing, and just blindly follows the trend, even if the tank and healer were horrible.

As some has said, the easiest way for a dps to get a com is to make yourself stand out, talk, joke all while still playing well.. And stray far away from making speeches in your macros