Why might a miner who controls the majority of CPU power still choose to mine honestly rather than attack?

The rules reward the biggest miner with more new coins than everyone else combined — so honest mining is more profitable than defrauding people and destroying the value of the wealth he just earned.

An attacker with more power than all honest nodes faces a choice: use it to generate coins honestly, or use it to double-spend and steal back his own payments.

Attacking would undermine confidence in the currency and devalue his own holdings. Since the protocol already pays the dominant miner the largest share of new coins, rational self-interest favours playing by the rules — the incentive design turns raw power toward securing the system, not wrecking it.