I don't know the reason, really, I don't. But looking at the reviews, Thor and Captain America was actually rated less than Ant-Man, not the other way around.

Ant-Man has received 7.3/10 on IMDb, 81% on Rotten Tomatoes.
Thor has received 7/10 on IMDb, 77% on Rotten Tomatoes.
And lastly, Captain America: The First Avenger has received 6.9/10 on IMDb, 80% on Rotten Tomatoes. It couldn't possibly be Captain America: Civil War, because the thread was posted before it was released.

Wow, Ant-Man has beaten one of the most famous superheroes of the generation right now.