In europe you do see a lot of convertibles and most of them are diesel, so I guess practicality isn't a real issue. Diesel is cheaper, car sounds like shit but it just drives like any other car.
It's important to remind you that most people are boring and they want boring cars, hence everywhere you go you see bloated suvs from korea and retarded trucks for normal (obese) americans.
And lets not forget that with the crazy weather and climate change (yes nigger) it's either too hot and humid to drive with the top down or too rainy/cold. Very few days it's actual pleasant to drive with the sun on you, or at night in the city.