While Ely is not as large as Reno or Las Vegas, it is a city with more population than other populated places in Nevada, like Panaca or Caliente. On a related note, Ely is a stop on US 93 when going from from Las Vegas to Idaho. The only other major stops along said route are Wells and Jackpot, neither of which are served by US 50.
Also, the wiki specifies that "trunk" is for the most important road routes. US 50, being designated a US highway and being divisible by ten, is a major cross-country route that spans multiple states. Even if it has been bypassed by Interstates like I-80, it is still an important road in the United States' road network, and I-80 would be classified as a motorway (freeway) anyway. This alone should make US 50 worthy of being a trunk route, while a one-mile-long US highway (like former US 94 in Florida) would obviously not qualify as a trunk road, even though it is classified as a US Highway.