Because there are people like Dennis Kenyon, John Beattie, the Black Cats, the Blue Eagles, the RAF Chinook Display, The Red Bull Bo105 doing some actually interesting "stuff that planes can't do"
There are many things that you can do in a rotary display that are interesting, but unless you can do something other than normal flying you have no business displaying.
A display is for the crowd, and should be interesting rather than just normal PPL stuff.
Everybody has youtube, and has seen exceptional pilots do amazing things. Dull routines ruin airshows.
At the very least, wingovers, noseovers etc and precision flying.
Unless you are flying something interesting in itself (MI-26,V22?), you have to do something that at least looks exciting or why bother?