[rsynnott]

To a large extent, because they're building established designs. An EPR plant (that is, the same design as this one) was completed in China in about half the time of the Finnish one, but that would have been informed by the problems in building the Finnish EPR, which was the first in the world. Another EPR, being built by EDF (a French company) in the UK is broadly on-track, and should have a much shorter time to switch-on than the Finnish one.

This isn't new; historically, the first couple of examples of any given nuclear power plant design have typically seen major overruns.