My guess is maintenance and longevity. We don’t have electric ships yet, so any tidal requires ships (Fossil fuel) for repairs. And salt water is incredibly corrosive, not to mention all the sea life that up has evolved over millions of years to stick to any surface.
When “the smart people” compare the maintenance needs along and power generation with off shore wind, (where the moving parts are above the water) my hunch is there there’s no comparison.
Also, speaking of that sea life…off shore wind farms have shown to crest new safe environments for fish. I doubt this would be the case for something that moves in the water.