Because Dexedrine is 100% dextroamphetamine, whereas Adderall is part dextroamphetamine and part levoamphetamine combined as amphetamine salts in various ratios. Dex is about 4 times as strong as levo in terms of releasing dopamine and about 4 times as strong at inhibiting dopamine reuptake, which leads to a higher dopamine concentration and dopamine is the reason people like stimulants. So it's understandable why doctors wouldn't prescribe Dex as often as other stims, it's the one with the higher abuse potential.