Tidal power involves machinery below the water, which then becomes as expensive as a boat to maintain. Wind power has just ridden the cost reduction curve so much better.

The tides are not constant but they are predictable.

The majority of tidal flow is during relative short periods of only a few hours. In order to harvest tidal energy, truly enormous volumes of reinforced marine concrete have to be deployed. The environmental consequences are massive.

Tidal power involves harnessing the power of corrosive sea water.

Once we work out how to do that in a cost effective way the UK is in a brilliant position. But until then it is an active area of research.

It's really hard to build good tidal power plants beyond the demonstrator stage. Most are difficult to build, expensive to maintain, and require a lot of maintenance.

https://www.orbitalmarine.com/ is one company working on it. As another comment states, working offshore unbelievably challenging (corrosion++, weather, position etc). Orbital's innovation is that the rotors move to a position where they can be serviced from the surface, rather than requiring any underwater activity, so it's a lot cheaper.

There are plans for big tidal barrage projects, especially in the Severn Estuary. Apart from creating power they'd also protect cities from rising sea levels.

They always run into Nimbys who don't understand that sea birds etc are far more threatened by the direct effects of climate change than by projects that affect their habitats in a designed way.

Tidal power is very, very expensive. It's more expensive even than nuclear power.