There's a handful of nuclear reactors compared to planes, and new nuclear reactor designs, where there's an opportunity to cut corners, are not being introduced at the same rate as new plane designs.
A reactor is a large, heavy, stationary thing. Economic concerns exist, but they're not going to make engineering decisions based on weight like you would in a plane where every kilogram of material costs a fortune in fuel over the lifetime of the plane. An extra chunk of concrete in a nuclear plant costs nothing, operationally speaking.
We're just lucky that the planes aren't nuclear despite many wildly ill-advised attempts to make this a reality.
There were updates suggested to modernize the facility, but for cost cutting purposes they were ignored:
It always comes down to cost cutting when the accidents are rare enough.
Newer designs have suffered more major faults and managed to contain virtually all of the radiation. American designs, in particular, place great emphasis on having an extremely resilient containment structure above the reactor. A lot of things can go horribly wrong but so long as the extremely radioactive gas is contained it can later be cleaned up. These radioactive elements are extremely toxic, but also very short lived. You just need to buy time.
The Fukushima design may as well have had a tin roof, it exploded almost immediately and exposed the reactor to the elements. If that's not a design flaw, I don't know what is.
That and a number of the systems necessary to keep the reactor under control depended on poorly positioned generators that weren't flood-proofed. This seems like a major oversight on a building located in a tsunami and typhoon zone.