Most proof of work coins use unique hashing algorithms and the miners are custom-designed ASICs that are extremely efficient at that one specific algorithm. If per chance, your coin uses the same algorithm as Bitcoin [ SHA-256(SHA-256(Block Header)) ], a small portion of the much larger Bitcoin mining community could turn their miners toward the smaller coin and quickly overwhelm their network to perform a 51% attack. The Bitcoin community can perform something like 40 billion gigahashes per second, most 'shitcoins' have total mining capabilities of 0.05x that if not less. It's a quick economic calculation for any miner or group of miners to decide to attack a smaller double-SHA-256 coin, as long as the total hashing power of the smaller coin's network is small enough and the coin valuable enough.