I think you mean to ask "why didn't they" since this has already happened multiple times in the past. The answer is that miners would have to take mining power

I think you mean to ask "why didn't they" since this has already happened multiple times in the past. The answer is that miners would have to take mining power away from the chain that they were mining. In practice the mining power that each chain receives is proportional to its current price. This is also something all the people upset about the electricity that bitcoin mining takes up need to understand. The electricity is being paid for by the people buying the balances that the miners mine. If people weren't buying the massive miner rewards every 10 minutes, the price of bitcoin would not stay the same and the same electricity usage would not be viable.