I'd have been far more impressed had an AI beaten some of the best wargamers at a complex wargame, or had it beaten some of the best text-adventure game players at novel text adventure games neither of them had played before.
The former would be difficult because of the enormous search space that results from the large variety of units, possible actions, and board size in complex wargames, and the latter is from the creativity required to do well at unfamiliar text-adventure games (which are hugely varied and can't be boiled down to any simple set of rules).[1]
Text adventure games would present a further difficulty in that not only do they not have very many players compared to games like chess, go, or Starcraft, but their players aren't normally ranked relative to one another, and there's no enormous record of human-played games to draw on. Further, in text adventures sometimes there are clear winning conditions and sometimes not, and sometimes clear signs of progress and sometimes not. It's also hard to objectively rate a text adventure game's difficulty. Without these features researchers would have a much harder time training an AI to do well against humans.
Another game which I briefly thought about nominating as a challenge for AI's was Factorio, due to its enormous complexity. However, when I thought about it for a little while I realized that this was a game that computers would find it easy to beat humans because they'd do a much better and faster job at number crunching and optimizing. Factorio is really more of a game where a human almost tries to emulate a computer, much like programmers do when they're mentally evaluating or stepping through an algorithm or trying to optimize something.
Of course, seeing an AI beat humans at any of these games would still make the front page of HN, but most gamers (nevermind most non-gamers) who would probably just say "What?" ... and that's probably yet another reason for the reasearchers choosing Starcraft. It's relatively easy, low-hanging publicity fruit, where they could claim success and a relatively large number of people would kind of understand why it was an accomplishment.
[1] - Yes, I'm aware of the recent HN post about fuzzing done on the Z-machines, but by the author's own admission that fuzzer cheated by having access to the game's word dictionary, and it also cheated by having access to the game's internal state -- neither of which a human player would have access to. Also, many Z-machine games are ancient and relatively simple and straightforward compared to more modern games.