Because you'd be limiting item diversity even more severely than it already is.
Shyvana for example gets a lot out of Trinity Force, but being manaless, she wouldn't be able to buy it. I'm not saying it's an always build item, but it has large benefits for split pushing, and when playing against teams without heavy hp stacking champions.
The Jungle items were restricted to smite, because otherwise everyone could pick up a Machete and clear some camps to get a gold bump from creatures, and initially they were trying to keep jungle items out of lanes, now a days with Cinderhulk and Runeglaive, I'm not so sure that's the goal, but they want to at least make sure that if you're going to take jungle items into lanes, you have to give something up for it. Your mid laner has smite instead of ignite, your top laner takes smite instead of flash. You get the picture.