Because people are mean and spiteful, especially around these parts.Why?
Sure, Rey is written as (yet another) insufferable, infallible girl boss, but I am struggling to come up with PR statements by Ridley (or even acting choices in those movies) to match.
Because people love watching celebrities die in movies.
Because she played the character that was central to, what a lot of fans considered at the time, the destruction of a beloved ip.
Because it would be fun watching her lizard face melt off from the fires of one of Doom’s weapons?
Biggest reason;
Because the Marvel Universe has ZER0 weight to it because no one dies, everyone exists infinitely in the multiverse so they are replaceable, or you can just bring them back as a different character. Having a presumably high profile character die, and stay dead, would add much needed weight. Kill what ever character Ridley plays, Captain Marvel, and a couple others, Doom using a couple infinity stones to do something diabolical, and have them killed in every iteration of the multiverse and stay dead. Kill off a few more over a couple of movies, and then have an Avengers Assemble for fighting Doom or whoever had been doing the killing.
But even better than that?
Because it would be funny.
(I am aware Black Widow died and stayed dead, until they made a prequel. However, she will return from the multiverse. Just wait and see.)