Whys this a stalemate?

The opponent king has no squares to moves if you promoted in a rook that's even not a stalemate but since you promoted in a queen the only square of h8 covers by your queen and his king has no where to go that's why this is stalemate
A game is won by Check mate, not by how much material one has. Over promoting often leads to a stalemate, which is what happened here. The King is not in check but can move to h8, so it is trapped on those two squares. You decided to promote to another Queen with a1=Q. That move stopped the King from being able to move to h8 and is not in check. Stalemate!

I had plenty queens and the kings weren't even close to each other
You don't know what "stalemate" means. That's the crux of the problem.
A game is won by Check mate, not by how much material one has. Over promoting often leads to a stalemate, which is what happened here. The King is not in check but can move to h8, so it is trapped on those two squares. You decided to promote to another Queen with a1=Q. That move stopped the King from being able to move to h8 and is not in check. Stalemate!
Aw man and I was close to winning
I had plenty queens and the kings weren't even close to each other