Why was there a 14 year gap between The Incredibles and The Incredibles 2?

For writer-director Brad Bird, it all came down to story.

“The thing is, many sequels are cash grabs,” Bird told reporters during a recent press event to promote Incredibles 2. “There’s a saying in the business that I can’t stand, where they go, ‘if you don’t make another one, you’re leaving money on the table.’ It’s like, money on the table is not what makes me get up in the morning; making something that people are gonna enjoy a hundred years from now, that’s what gets me up. So if it were a cash grab, we would not have taken fourteen years – it makes no financial sense to wait this long – it’s purely that we had a story we wanted to tell.”

Bird admitted that the proliferation of superhero properties in recent years has made it much harder to tell an original story; the first film debuted before Pixar’s parent company, Disney, purchased Marvel, and predated Iron Man by four years - meaning that the new film is entering a very different cinematic landscape from its predecessor.

There's a fair bit more in the source article

Why Did Incredibles 2 Take So Long? Brad Bird Explains the Sequel's Delay

Some of which also relates to multiple rewrites concerning the villain storyline and trying to make I2 more unique in an ever-proliferating cinema-scape of super-hero movies.

The ending of The Incredibles was never meant as a pointer to a sequel. As Brad Bird — who wrote and directed the movie — told CinemaBlend:

How did that Pixar adventure end? The family was leaving Dash's track meet, when suddenly, a former nemesis -- The Underminer (John Ratzenberger) -- bursts out of the pavement and declares war. But, in an exclusive interview, Bird told CinemaBlend that this wasn't always his planned ending, and he explained:

I had another ending that was kind half baked, meaning not really finished in my mind. And I knew that the way to classically end a film is show people getting back in the saddle and riding off into the sunset. But I resisted a little. And finally, my head of story on that film, a guy named Mark Andrews, said, 'Come on, you know what you want!' And I was like, 'All right, all right.' So I thought, it's a way of showing they're together, and that they're going to face whatever obstacles as a group, now embracing their superpowers. So I thought that's what was important to communicate.

I wouldn't have set it up so everyone wonders who The Underminer is. No, no. It was just, this will be a satisfying ending to this film. And if there's never another one, we get that the family's together, they're embracing their powers, and they're fighting whatever obstacle comes their way.

much like the first film, Incredibles 2 will explore "the roles of men and women; the importance of fathers participating; the importance of allowing women to also express themselves through work, and that they’re just as vital as men are. And there’s aspects of being controlled by screens. There’s feelings about the difficulties of parenthood, that parenting is a heroic act." – Interview with director Brad Bird

14 years is how long it took their target audience's children to have kids of their own. That was necessary for the new audience to appreciate the movie... and of course: extra ticket sales.