Personally I would assume because it’s a pain in the ass to maintain, when fossil fuels are less so and not presently heavily penalized.

I mean really, wind was the original seafaring option, so we already know it can be harnessed that way, but the current capitalist framework rewards doing things cheap at the expense of the planet.

Cargo ships use real bad fuels, anything would help, it just needs to be required or cheaper than polluting alternatives.

Looks like internalized costs for this rather than the externalized costs for fossil fuels. Only regulation can truly fix that.