[Alupis]

> The reason we sent a U2 Spy plane of all things after it is because it’s about the only plane we have still in service that can fly all the way up to 60,000 ft where the balloon was at the time

Additionally, flying high requires flying fast, generally.

The U2's huge wingspan allows it to fly much slower at these altitudes, and with much more ease and efficiency than say, an F-15 or F-22 as some other commenters have suggested being capable of shadowing the balloon at FL600+.