it mast be simple because most of them were found in that area …..

Deserts were once dense jungles, full of life.
Deserts preserve bones anfd fossils well.

America and China have vast deserts.

Dinosaurs lived (relatively) globally during the Mesozoic Era (even the present day Arctic had proto-crocodiles living there), so it certainly shouldn't be that they were concentrated in these areas. I would make an educated guess that either

Spectacular initial finds in these areas spurred more exploration, so efforts were concentrated in these regions.

Or more likely, the potential to be preserved was particularly higher in these regions that in other parts of the world. Preserving a fossil is incredibly difficult, let alone a whole dinosaur. Preserving marine life is hard enough, terrestrial life accounts for much less of the fossil record.

Additional information - A desert wasn't necessarily a jungle, previously. A desert can evolve from any landscape.

The two regions in question are remote deserts or mountainous areas. These tend to be undisturbed, since there had never been anyone there looking for dinosaurs.

(I, too, remember the stories about Roy Chapman Andrews' expedition to Outer Mongolia.)

I remember passing Tucumcari, New Mexico and seeing the signs: "Walk Where the Dinosaurs Walked".

What you have to remember is that they walked everywhere!