Well, something happened to the franchise. It shifted focus from car racing to action scenes. This was good for the box office but there is little remaining for car enthusiasts. The reason the Fast and the Furious originally appealed is that chase scenes, races, and awesome cars were the focus. The vehicle action scenes almost seem forced in the later movies.
Bloomberg posted an excellent piece detailing how things changes. Let's just take a look at a few of their data points:
You can read the full (and very well done) article here: https://www.bloomberg.com/graphics/2...t-and-furious/
At least there is room for a new movie or even franchise that is more centered around cars as the Fast and the Furious moved toward big budget action scenes and Hollywood starpower.