Because you along with so many players feel like Riot owes us URF for whatever reason. It was originally once a year during April got a while. We then complained we wanted more. We then had them change it up a bit and put it as a featured game mode for a few different weekends over the course of a year. We complained we wanted more.
People won't be satisfied until urf is a permanent game mode. League markets is game as a 5v5 traditional moba. By making urf permanent, you'll both take from the natural game and also create a dilemma where people will want to treat it as a real mode with real balance. People already complain about OP things in urf in the few days it comes out and want balance in a game mode designed to be horribly unbalanced.
It's not an attack on you, it's an attack on the attitude of so many players that won't be satisfied until it's an official mode such as Aram.