No explicit reason, they just generally don't. When you're beholden to voters or a city council, your job is least threatened when you don't rock the boat. If you do rock it, you better make sure you're right or it's not likely to end well for you. Meanwhile, your 'subjects' are unwitting participants who thought they'd be getting a normal education, not early adopters explicitly willing to try something new. All of these factors discourage experimentation.