> while seven U-2s were lost during training missions, killing six pilots

..out of 19. Is this an exceptionally difficult aircraft to learn how to fly?

Yes. Other comments in this thread have pointed out that the airplane has a very narrow window of acceptable speed that's not too fast (overspeed, damages the plane) and too slow (stall, causes the plane to drop out of the sky).

Which makes me think - what was the motivation to be a pilot of one of them, if the risk is so absurdly high? Prestige of being one of the very few certified to fly it? Did it come with better compensation? Or could you be ordered to fly it if you were good enough, no choice given? Or a combination of all of these?

Mostly it is military, so you pass flight school and then get your orders. Sometimes you can ask for a specific assignment, if it isn't in demand and isn't below you, you can get it, but part of asking is knowing who to ask and how to ask for it. (My Uncle was in the Navy and he got a lot of choice assignments because he asked for them and most of his peers just went where they were ordered)

Might also add true believer (if someone doesn't do this the commies will end my way of life). And, from what I've read more likely, "my buddy will die for sure in that thing, at least I have a chance."

Too fast also causes the plane to drop out of the sky, as you start getting supersonic airflow over the wing causing it to lose lift AFAIK.

I've seen one of the crashed U2 in the PLA museum in Beijing.

It's a very interesting museum, well worth a visit.

s/crashed/shot-down