In a cold climate, I would expect burying it to use the ground as a natural insulator. Why was an above ground design chosen?

Specifically, does the need for heavy insulation and the active heating of the sand make the ground a less effective or even problematic insulator? Could excavating and building a below-ground foundation for a high-temperature device like this be more complex and expensive than an above-ground silo? How would permafrost conditions affect this design?

> Why was an above ground design chosen?

Because digging is expensive and there's plenty of land. More efficient to use the budget to build a bigger structure than to build a smaller one and dig down. Bigger structure also gives you better insulation (surface area compared to volume decreases non-linearly with increased volume).