Because of so many reasons....
-They can easily favour Indians and Philippines (their own nations)

-Most of time they can work cheap and in adverse conditions

-There is no any rule in Qatar, everything depend on favour/reference (WASTA), they are everywhere so imagine who will get hired

-Qatari don't care as long as you are their slave- matter how much you are educated or skilled, they are best at these

-Qatari likes people who can work cheap and can become their slave...

-There is no value of talent, skills and experience

-there is no any test, interview or selection process...

And many more.....