Because they're expensive and unreliable for most applications. They need more power, they increase draft, they don't tolerate debris, and they really dislike rough seas.
They do fine for passenger traffic. Draft, debris and rough seas are not a problem. You need gas turbines or very unusual diesels for the power, that is the problem.
I don't know why hydrofoils aren't more popular
Because they're expensive and unreliable for most applications. They need more power, they increase draft, they don't tolerate debris, and they really dislike rough seas.
They do fine for passenger traffic. Draft, debris and rough seas are not a problem. You need gas turbines or very unusual diesels for the power, that is the problem.