Not the only answer, as this article explains, but it’s one of the biggest factors.
Choosing a route and paying for the land are two parts of the same task.
Probably a good time to remind Seattleites that Paris — which takes up half the land as Seattle with 3 times the population — has several large railway stations — Gare du Nord, Gare de Lyon, Gare de l’est — within that footprint as well as the Métro and the RER lines. Not all of them date back to the 19th Century.
It’s never been clear why Seattle needed such large coaches, with the requirement of larger tunnels. It’s also understood that cut and cover tunnels, was were used in Paris for a lot of the Métro, wouldn’t do here. But why so large? They seem to be as large as a freight locomotive.
But the biggest issue is the land used for stations, the value (and price) of which will rise dramatically as it get puts to a higher and better use through density and development. Because we have a land cartel instead of a land monopoly, we are forced to pay high prices for land, rewarding speculators at public expense and delaying action on what really should be seen as a war on climate change and housing affordability. But we have decided to support a cartel of land owners vs managing it for the benefit of everyone.