Incentive and 51% attack
Bitcoin has a huge incentive system to discourage a 51% attack. Bitcoin mining with CPUs is impractical. You have to use dedicated hardware that can only be used to mine Bitcoin and other coins that use the exact same algorithm.

So to acquire enough hashing power to launch a 51% attack on Bitcoin, you can't just rent computing power or buy CPUs. You have to invest in Bitcoin mining equipment. If you manage to harm Bitcoin significantly, you're just turning your expensive mining hardware into so many doorstops.