The Ruy Lopez is one of the openings with the best reputation. For that reason it is often played. For that reason there exists more theory about it.
Apart from the Ruy Lopez and the Grünfeld, also the Sicilian and the King's Indian are heavy on theory.
Why is Ruy Lopez so theory dense
Sort:
Jul 3, 2021
0
#3
The easy answer is because a lot of strong players have played the Ruy, so a lot of knowledge has accumulated.
The more difficult question/answer is why do GMs prefer some openings over others, and what sorts of positions tend to have more theory.
The answer to that should probably include something about positions that are both unbalanced (in terms of pawn structure and otherwise) and objectively equal.
For example in the position below, white to move, GMs have tried practically every legal move 
. So there is a lot of theory. You can tell from the structure it's unbalanced, and you can tell from how many different moves white has tried that black is always finding ways to equalize.
-
-
At my level, you get people playing the Ruy thinking,"oh, Bxc6 dxc6 Nxe5, oh, i'm so smart, I removed the defender and won a pawn" yeah no you idiot Qd4 fork you just lost your right to castle I have advantage now, but at much higher levels, you find massive amounts of theory, and i think the only competitor in terms of theoretical density is the Grunfeld defense. why, in e4 positions, does the spanish have so much more theory to it, than, say, the italian or petroff?