The Jetsons Meet the Flintstones (1987)
The time machine didn't transfer The Jetsons to the past, but to the future. The world of The Jetsons is an advanced futuristic era, with cities set above clouds... but we never see the ground.
The only explanation is that the Earth was devastated by something (war, virus, plague...) and that's why people are living 'in the sky'.
So, according to that theory, The Flintstones is set on Earth, after the society of The Jetsons crumbled/left to space - that's why we have TALKING dinosaurs & prehistoric animals, coexisting with humans - evolution/mutation did that.
That's also why there are TV sets, automobiles and other 'modern' equipment in the 'stone age'.