Mainly because most items are ridiculously overpowered...some items are just plain useless though, so I guess it balances out a bit. But it always makes you angry whenever you get hit with a fan 500 times, or you get hit by a bumper thrown at you, getting hit repeatedly by someone's Home Run Bat they just threw, getting your Bob-Omb reflected back at you when you try to throw it, or throwing a Smart Bomb too close and hitting your opponent AND yourself...the Ray Gun, in my opinion, seems to have been the item that has been buffed up the most. No longer can you dodge the blasts between each blast, now you can keep getting shot until you get hit off the side of the screen.
Mr. Saturn, Smoke Balls, Springs, those are items that amusing and fun top lay with sometimes, but overall, I'd rather keep the items off in a match.