How do new Cryptocurrencies prevent a 51% attack?
One possible reason to not attack a coin with weaker security is the lost opportunity cost. In order to attack a network, the attacker will need to devote hashing power to the attack. 

If they did not stage an attack, they could use that hashing power to mine a profitable coin instead. The benefit of the attack must at least outweigh the lost opportunity cost of engaging in the attack.