Because he is at higher levels, perhaps? Do you think we'd even be discussing this if it weren't true? The light arguement has been made too many times and people who can kill him off the top as low as 70% (on most stages using the right attacks) like Donkey Kong, Bowser, Ike, Wario, Luigi, Fox, and Snake are still 60-40 or worse. Olimar vs Metaknight is still 70-30 in Metaknight's favor because if Metaknight gets Olimar in the air or off the stage, he's pretty much screwed. Also, Metaknight can use the tornado to slightly hover and attack over the pikmin's grab range. Regarding your situation, Metaknight can outprioritize Olimar's aerials with all his aerials and all his specials except the dimensional cape. Also, in higher levels of play, grabs become less frequent because the players are more knowledgable about how to avoid them so depending exclusively as a winning strategy is not a good idea.