It's true of almost all ICOs launching ERC20 tokens because on Ethereum you can write code that automatically turns your ETH into ERC20 tokens. A couple pages of code and it all just happens on the blockchain by itself.

If you take bitcoin contributions, you need something offchain to track who contributed what, and issue a bunch of minting transactions.

Some ICOs do that, but if it's ERC20 that's generally in addition to automated minting via smart contract, and since the token and ICO are running on the Ethereum blockchain it's predominantly ETH that gets contributed.

Liquidity on exchanges later doesn't have much to do with any of that.