How come there was never a Days of Thunder sequel?

Because the film was disappointing at the Box Office:

https://www.latimes.com/archives/la-xpm-1990-07-02-ca-445-story.html [‘Thunder’ Sputters in Box-Office Race]

"The Tom Cruise stock car racing film “Days of Thunder,” a movie that falls somewhere between a remake of and a sequel to Paramount’s 1986 blockbuster “Top Gun,” got off to a surprisingly slow start."

In fact:

https://www.latimes.com/archives/la-xpm-1990-11-16-fi-4799-story.html [2 of Paramount’s Costliest Top Guns Lose Their Jobs]

"The official reason given for the split was that the “environment for motion picture production has changed.” Sources close to the discussions, however, said relations between the two sides soured over two key points: the poor box-office showing and budget overruns on “Days of Thunder” and the producers’ failure to develop many projects in recent years."

Troubled production!