Why insulin reacts only on glucose

because the body produces glucose. Its the simplest monosaccharide. Fructose has a different molecular and xline structure and the others like sucrose are disaccharides and on and on.

they all break down into glucose (or like fructose just go through the system as is), .

Transport of other things by insulin besides glucose is not natural?

Im going to ask you to look up Glucose to pyruvate(Sugar CATABOLISM). (Its a breakdown of complex to simple sugars).
Theres only 3 pathways this occurs and I think we have some real microbiologists on the board who can explain this much better than I.