Probably the same reason we don’t go to other kinds of races. Why watch a tiny part of the whole? I have been to the Indy 500. Fun but could barely make out car numbers as they drove past.
As to participation, I run a half marathon once in a while, mostly to keep me from slacking though I place fairly well. I would ride a bike race just for a good coffee, even though I would be in the back of the pack. But as to participation, why pay when I can do almost the same for free? I don’t need another medal or whatever, I have a box at home and still wearing years worth of race shirts.