These other cryptocurrencies are less secure than bitcoin.<p>For them, the cost of attacking the network by mining on an orphaned chain is lower, because by definition the PoW has a useful by-product.<p>The more useful the byproduct, the less secure the coin.<p>Edit: to illustrate, imagine a PoW that computes cures for cancer. Somebody can now mine on whichever block of the chain he wants forever, since he&#x27;s producing a cancer cure anyway. The incentive of a block reward (within the currency) is not primary. Thus there is diminished incentive for the network to create a linear blockchain, where everybody mines on the  most recent block. Without that, you cannot order transactions.