They're specifically using old-style techniques to build it, and doing a sort of fund-as-you-go structure where donations finance the construction. This is combined with the fact that the building is really complicated and every block has to be custom-carved into a unique shape.

There's also likely an element of not really wanting it to be done since the construction itself is a big part of the draw of the building for tourism.

And yet the author of the present article would simply judge it by weight and say "with certainty" it must have taken only a fraction of the time.

I hadn't heard of it, it seen pictures, until we went to see it in Spain. The building is absolutely mind boggling. The sort of thing that makes you proud to be a human.

When you see it, the timeline starts to make sense.

Funding but also they are to an extent a giant work of art, adding more people does not produce a better work of art.

Funding has been the limiting factor, as far as I know. There have been long breaks where no work was being done, and it was damaged during the civil war. Also the building is immense and very detailed.

The fact that Gaudi's original plans were lost in a fire don't help