In fairness, every single Jurassic Park sequel to date has struggled to come up with a plot that convincingly justified its own existence, and Fallen Kingdom is certainly no exception.
The Lost World conjured up Isla Sorna out of thin air, Jurassic Park III brought Alan Grant (Sam Neill) back to Isla Nublar with dubious plausibility, and Jurassic World tried to convince us that people were bored of dinosaurs. Hmmm.
Fallen Kingdom instead opts for an animal rights angle, whereby humanity has grown fond of the genetically-engineered dinosaurs and passionately campaigned for them to be rescued from Isla Nublar before a newly-active volcano renders them extinct (again).
Sadly, the movie does a horrible job selling this to the audience: it's mostly just vomited at the viewer in a quick news reel exposition dump, papering over the fact that the dinosaurs slaughtered hundreds of people in Jurassic World.
That's to say nothing of the fact that the island's long-dormant volcano conveniently becomes active. Perhaps it wasn't such a great idea to build a dinosaur park next to a volcano which could switch back on at any moment? Some expense was clearly spared somewhere.