Boat-tailed cars were sporty looking due to that style as well as because usually they were roadsters or convertible coupes that tend to be intrinsically sporty.
A major problem with boat-tailed cars was lack of space for luggage; non-boat-tail roadsters and convertibles were more practical, and sold better. So the style died out.
But a few echoes of it appeared now and then on American cars. In these cases, the cars' rear ends didn't have boat shapes. Instead, the aft part of the passenger greenhouse or perhaps sheet metal sculpting on the trunk featured convergence in a sort of tribute to the boat-tail. Below are some (perhaps most of the) examples.
This exaggerated, aerial view of the Auburn Speedster proclaims the boat-tail's spirit.
The actual car was a lot shorter, but very attractive. It was a facelift designed by the great Gordon Buehrig.
Rear view of a 852 Speedster showing its boat-tail.
The Studebaker Starlight Coupe first appeared for the 1947 model year, creating a sensation due to its then-futuristic appearance. Note the converging raised area extending from the passenger compartment over the trunk. Not a boat-tail, but the spirit is evoked.
The upper part of this Buick fastback converges considerably, though not to a point -- yet another boat-tail echo. General Motors fastbacks of the late 1940s lacked the trunk room of GM bustle-backs, so the style was dropped in the early 1950s.
Bill Mitchell was head of GM styling starting in 1958, and the Sting Ray was one of his pet projects. The greenhouse converges to a point in plan view, much in the boat-tail manner. Note that the rear fenders do the same.
Another Mitchell-inspired design. Here there actually is a boat tail, stubby though it might be. The overall design is awkward, however.