All of this.
Tons of actors are great at emoting, not acting. They don't act like normal fucking people and their performances override the character they're supposed to be portraying.
Keanu doesn't do that shit. Doesn't mean he's a bad actor. It means he's humble enough to not make the entire movie about him. Which makes him relatable.
He's arguably one of the greatest action movie actors ever. Putting aside the insane amount of work he puts in to learn martial arts, use weapons, etc., his acting style allows the audience to put themselves in his shoes as the lead. You make the Matrix with anyone else and it probably fucking sucks (I think Will Smith was in the running at one point and hoo boy that would have been a trainwreck). Keanu plays stuck, bored, corporate slacker/malcontent to a T and then perfectly encapsulates what all of us would be like if we got sucked out into a dystopia where robots use humans as batteries.
Oh, and unlike all the other kiddie diddling creeps and assholes in Hollywood, he's a good dude. Gave away a ton of his money to the people that worked on the Matrix, IIRC. He's like #2 behind Tom Hanks as all-time nice guys in show business.