When it is 42 deg and raining sideways, any air or water leak will become immediately apparent. There are many parts of this country where the weather is miserable for 6 months a year or more.
Most speedster owners compromise a good deal of weather protection for the near-perfection that comes when the car is cruising along with the top down. The top never comes down on a coupe, so the calculus changes. The world isn't clamoring for a leaky coupe, since a buyer is sacrificing the best part of owning a replica (the top down part) so he can actually use the car.
A coupe buyer presupposes weather protection. What other possible reason is there for buying one?