Can't use your arms to help you blade uphill. Bikes cost less to maintain. Aluminum roller blades would be stupid. There are no plastic bicycles. If you hike into the mountains, there are no roller-bladers.
Bicyclists outnumber roller-bladers.
Riding roller-blades in the street is a bad idea. Riding roller-blades on the sidewalk is a bad idea. No such thing as a roller-blade pump. No such thing as roller-blade shoes. A bicycle u-lock makes an excellent weapon.