Many reasons, the first few I can think of are: 1) It's not a _few_ hundred, it happens to be exactly 100 in Nevada. 2) That was spread over many years vs a couple days of a war. 3) They were in a barren desert, the "safest" place we could find. 4) They were all early bombs, nothing compared to the modern stuff. H-bombs literally use nukes as detonators, so it's like comparing blasting caps to C4.

We quickly realized this was a terrible idea(some tests in NV had intense fallout in NY) and stopped doing atmospheric tests by 1963.