2D sidescrollers most commonly have four directional buttons plus two command buttons. One of the command buttons is for jumping while the other performs actions like shooting. If you wanted to have something like dashing you'd have to add a third button or add a double-tap sensor to your directional buttons. Adding buttons to cycle through items or to open an inventory screen further complicates things. Having yet another button to use said item makes things even more unwieldy.
It seems like most PC gamers have some way of playing games with a controller, but when designing your game to use many buttons
it can come out like a mess for someone stuck with a keyboard. Just how many buttons is too many?