In my experience it's because schools are being treated as a business, and businesses are usually more efficient when there's consolidation of expenses. Why pay for 3 schools with 10 teachers each when you could instead consolidate classes and pay for 1 school with 15 teachers? To a business, the decision is purely made out of cost. Alas, a lot of governments have such tight budgets (for many legitimate and illegitimate reasons) that cost benefits outweigh the human benefits.