ok jets have an airfrom that is not very stable ie it will not come to "wings leval" if you leave it alone in lets say a 20 degree bank a trainer will go to wings leval and is very stable you can make all kinda of mistakes with trairs and they forgiving now a jet will keep the 20 degree untill you tell him you want him to lvl (opppoosit imput requird) this increases poilit work load now your not thinking about were you sould be going but more the "how the hell do i keep this in the sky" you mannick and it comes down like a rock and you just lost 300$

Also jets generat less drag so the same package will move considrably faster there is also wing load what is the wing load the higher the wing load the faster you have too land and jets as a rulle oof thum ALWAYS have a relitivly high wing load

there ar emany other reasons but those are the main ones i could think of