Costs are prohibitive. I don't think the people who suggest this have any concept of how over-stretched school budgets already ARE. Teachers are paid crap, the school runs with partial staff, and teachers have to often buy classroom materials out of their own pockets. Who is going to pay for laptops for every student? TExtbooks can be used for a much longer period of time and go through multiple students.
Of course, thinking you can get everything you need to know online is naive. You'd still need to have reputable sites with as much information as a textbook- which you would only get if textbook makers started publishing online content, which you'd then also have to pay for.
You also have the problem of damage and loss- if the kid's entire curriculum is on one computer that they must haul everywhere, how many are going to be damaged and need replacing?