Because there's no dynamic to keep things in check. It's natural that one browser has more market share for one reason or another, which results in webpage owne

Because there's no dynamic to keep things in check. It's natural that one browser has more market share for one reason or another, which results in webpage owners looking out more for things functioning in that browser properly, which in turn leads to more users using that browser. Same reason why the operating system market and the processor architecture market have virtually no competition and an objectively terrible product in a leading position.