Never seen a car with roller-blades strapped to it's roof. Nobody ever rode a bike to disco music. A triathlon does not include blading. You can still ride a bike that isn't your size. 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.