Blame the Flavians.
When they built the premier sports facility for their games, they commissioned a round bowl with premium seating for the wealthy, and a roof to keep the sun off the heads of the ordinary folk. It became the ideal for what a stadium should look like.
The design had its deficiencies, of course. The Flavian Amphitheater (better known as the Colosseum of Rome) wasn't exactly accessible. The steps to the cheap seats were twice the size of modern steps, and there were no handrails. But it was a marvel of the age, and probably kept the Gladiators from moving to Los Angeles.
Centuries later, when the roofs rose over the famed — or infamous — facilities of the '60s and '70s (the Astrodome, the Silverdome, and yeah, the Metrodome in 1982) the style was set. Who would ever change it?
Obviously, the architects of U.S. Bank Stadium were not slaves to history, which is why the angular shards of the new building are shocking to some. To understand why it's a novel design, consider the two approaches in modern stadium design: the Bowl and the Not-Bowl.
When you think of a bowl-shaped stadium, what comes to mind? The Metrodome. The charmless concrete pot with the marshmallow roof, a place so dour they painted some of the struts to make it seem happy and exciting. It was an utterly unadventurous structure, but we came to love it — partly because the Twins won there, and partly because it was ours. But it also had a unique characteristic: The acoustics made it possible to deafen the opposing team and ensure their ears would ring for a day afterward.
If we seemed indifferent to the banality of the design, it's because that's what stadiums were supposed to look like. When the Dome was built, the faux-historical style of baseball fields like Camden Yards was a decade away. The Dome was in a lineage of big bowls stretching back to the Astrodome. In 1965, it was a marvel of the modern age, and its climate-controlled interior made outdoor facilities like our own Met Stadium look like 19th century relics. Every city wanted one.
Until they started to look the same, that is. It became clear that the Bowl needed an updating, and the 21st century provided many variants. The entire exterior of Allianz Arena in Munich can change colors, depending on who's playing. Stadion Energa Gdansk in Poland is clad in varying shades of gold, and Juventus Stadium in Turin looks like a glass-clad flying saucer.