The board is not symmetrical about the horizontal axis. In other words there's a kingside and a queenside. It's the same reason the king's gambit is far less popular than the queen's gambit, or why kingside castling is much more common than queenside castling.
g6 lets you castle faster, and the e4 d5 structure "points" at the queenside.
b6 makes castling slower, and the d4 e5 structure makes your king less safe.
That's how I see it anyway.
Just a random thought I had perusing openings in analysis. The Modern Defense has about 55,300 games played in the database with ~36-26-36 WDL for Black, whereas the Owen Defense has only 3,800 with a ~31-22-47 WDL for Black.
Are there any glaring issues with the Owen Defense that would explain this disparity? Obviously both issues are quite similar at a glance being symmetrical Fianchetto openings. The first thing that comes to mind is that the Queenside Fianchetto doesn’t let out the F file bishop to enable Kingside Castling? But I feel like there has to be more than that to explain such a large disparity.