With Bitcoin, if two miners create a block at about the same time, only one of them gets any reward for it, and only one contributes to tthe total proof of work on the chain. So Bitcoin has to make blocks less frequently so this doesn't happen much.

Bigger blocks take longer to propagate around the network, so a miner is more likely to build on a block without knowing it's already too late. That's why Bitcoin-style chains can't make their blocks too big.

With Ethereum, when two miners build a block at the same time we set one as the main block, but still pay for the other as an "uncle," and when we choose which blocks are official we take the uncles into account, so they contribute to security. That's why we can get away with 15-second blocks and more tx/sec.