The cynical view would be that during that time they left the government twice to protest the government approving new nuclear power projects, and paid a heavy political price for that. They can't afford to do it a third time, and given Finland's position (can't rely on solar or wind during winter, can't build more hydro, can't depend on imports during the production troughs) it is highly likely that more nuclear will be approved at some point.

The non-cynical view is that the world has actually changed over that time. Reducing CO2 emissions is more urgent than ever, and there's no longer time to hope that we can instead reduce energy usage which used to be their preferred solution.