Because it's very hard to do. The Sea is just an incredibly hostile environment in which to put generators. People have been trying to develop the technology to do it for decades and there is some promising stuff on the horizon but it isn't there yet.
How did France manage to do it on their Atlantic coast, then?
I assume you mean the Rance Tidal Plant? Yes, there are some tidal plants around the world: but they're small scale, largely on rivers rather than off-shore (meaning potential sites are highly limited), and costly compared to other renewals All that needs to be solved for it to play a large role
With the current state of engineering in this world, I refuse to accept that it's beyond current technology. We can achieve great things with the right amount of willpower.
People are trying. Hopefully we'll get there :)