When you want to import a bitcoin wallet on trustwallet, there is only one option which is secret phrase.
That doesn't make sense, but then again, so much what Trust Wallet is doing doesn't make sense. If they cared about their customers (which they don't), one could argue that they don't allow importing individual private keys to help in preventing address reuse for privacy reasons. But that's surely not it. Ethereum and its tokens are all on the same address, so it doesn't make much difference whether you have a seed or private key. Except that the former is more user-friendly to back up than the latter.