Because of the women. Yes, they can't resist the ROTJ.
Okay, no, I'm doing a SE just because that's what says Boba to me. The ROTJ outfit looks closer to the Prepro 2, which is how most of us first saw him, and yet it's a costume that was actually seen on screen. Add to this the better helmet and you've got the SE.
I hear all the time how people don't like the technicolor scheme, but that's how he was originally envisioned . . . having "borrowed" different bits and pieces from different places.
The all green ESB scheme is kind of blah. I also prefer the more intricate blaster in ROTJ. Always wished the prepro sling blaster had seen screen time too, though.