Three reasons:

1. Concealed means "obscured from view" so OWB is concealed enough. I don't need more concealed than that.

2. IWB feels less secure to me. When I sit down it feels like the gun is trying to climb out the top of the holster. OWB in a nice leather pancake style holster is completely secure, even if I should become inverted for some reason.

3. OWB is FAR more comfortable when sitting, standing, or bent at the middle.