Presumably because carro is the more common word in the majority of Latin America (auto is used in some South American countries instead of carro). Coche is only used in Spain, a small portion of Mexico and Equatorial Guinea. Duo teaches Spanish that is more representative of Latin America (mostly preferring neutral Mexican with some variations), so using the most commonly used Latin American word for car makes sense. This doesn't explain why they've suddenly switched to preferring patata over papa in some parts of the course, though.
I wondered the same thing, although I did note that "carro rápido" is more fun to say than "coche rápido"!