... other than maybe because he's hard to get?
I'm a fairly new player and just discovered there was a Rhaegar Targaryen card printed in Battle of the Ruby Ford, but looking around the different deck builds on CardgameDB, i realised no one actually plays him. Not sure why, as he looks pretty decent for a 4 Gold, 5 STR TRICON character with TWO crests. And yes, he can end the round if he dies, so if you get the first go and do all your challenges only to let him die, then essentially your opponent doesn't get to go for a challenge. What's the big drawback?
Either I'm not reading his card right, or people are finding other stuff more useful to play.