I've thought about this before because even as someone who's loved them since the first film,the franchise success really is insane.
But I think it comes down to a lot of things.
One,the first film was a cult classic especially with young people. The film single handedly created and pushed a bigger car culture back then..everybody wanted a suped up car.
Not only the movie was just quotable. Everybody could recite it line for line.
The 2nd one cashed in the first success and brought in new Characters but it didn't hit with people the same way.
The third was basically a reboot and the franchise well on its way to being straight to dvd status.
But then Vin and Paul had a string of misses and came back. People came back to the series for the fourth for the nostalgia even if it was a weak film.
And then a weird thing happened.
The 5th came put, they added rock,and the flat out made one of the best action films in the last couple decades.
Not only was the action great,but they respected their audience. The audience that grew up with the franchise and stuck with it through the lows. Instead of being ashamed of past movies,they made them more integral. They brought back characters left and right. They wrote in sub plots to connect the film's even deeper. Yes it's all silly but it respected the universe it built.
One of the best things about the franchise is them constantly mixing characters events of past films.
So you had great action,mixed with likeable characters and writing that respected the audiences Fandom and rewarded it.
On top of that,these films fly against the nature of the overly cgi green screen action films now. These don't have tacked on 3d, and they do real stunts with real cars.
The cherry on top is the diversity and the celebration of the world by having so many different races and religions represented. What other franchise can appeal to so many bases? They filmed this one in Cuba in parts. They film all over the world. Show different cultures instead of the same white male tropes. The cast is diverse as it gets.
It delivers everything it tells you its going to while celebrating diversity.