Because they're sissies and think it's too big for the street. It can work fine, but you'll simply have closer to a race bred engine on the street. With the proper converter and rear gears and all the other matching components, it'll run great. It IS a big cam, but people run .750 lift rollers on the street everyday. It CAN be made to work and work well.
What it AINT gonna do is run smooth at the drive in or get good mileage.