a) They are not very expensive
b) Given how popular they are, there's no other pair of monitors any given user is more likely to have used before... Meaning they are a "familiar reproduction baseline" for musicians, producers and engineers who are often working outside of their own studios and have to make critical decisions on the spot.
In recent decades, as audio monitoring fidelity dramatically improved, NS10s also became increasingly useful as secondary monitoring devices. "My mix sounds great on these hi-fi speakers — let me see if it also translates well to my ("crappier") NS10s!"
Finally, for anyone looking to get serious about audio monitoring for an arguably reasonable price (compared to other pro options), I highly recommend monitors by HEDD, a Berlin-based company founded by physicist Heinz Klaus (formerly R&D lead at ADAM, another solid audio monitor manufacturer whose products I also used for years). Here's Sound on Sound's review of the pair I currently own: https://www.soundonsound.com/reviews/hedd-type-20