How do new Cryptocurrencies prevent a 51% attack? I've seen similar questions, but not specifically in reference to new altcoins.

When a new cryptocurrency is released, the developer(s) have limited amounts of resources. What prevents a group from carrying out a 51% attack while the mining network is still small?

I realize there isn't likely any benefit to the group carrying out the attack, since coin value will subsequently be lost, so the motivation would probably be carnage or the desire of eliminating a rival coin.
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.