They were nowhere near poor choices. But at the time of their respective Predator movies, they weren't A-list actors though. Brody was once upon a time, but only briefly and that was for a couple of years after his Oscar win. He's been struggling ever since and he needed Predators more than it needed him. And all Glover had was two Lethal Weapons(second fiddle to Mad Max), and supporting roles in Witness and The Color Purple. Great movies, great roles, but... He hadn't proven himself as someone that could open a movie. And Predator 2's box office didn't help his cause. Now if Fox had chosen Bruce Willis or Mel Gibson for the role, then that would've been an A-list choice. But we know they wanted to keep costs down, because they were also contemplating using Seagal.
An A-list choice would be someone like, well, Affleck or Cooper. An actor who doesn't need the movie but stars in the movie. Both of these men have shown they have the capacity to open a movie, provided the movie is good. The Town, Argo, American Sniper, etc.
Let's even look at the first Predator. Arnold didn't need Predator, he already had Conan the Barbarian and The Terminator. He was a Golden Globe winner. But yes, Glover and Brody are fine actors. There's no denying that. But they're character actors, not leading men. So it's not a question of being a great actor, but being an A-lister.