My impression was that dextroamphetamine also worked better in many patients than drugs that combine it with levoamphetamine such as Adderall. Even when Vyvanse was under patent and expensive, it was preferred by many for this reason (Vyvanse is converted into dextroamphetamine in the body). Levoamphetamine primarily acts on the norepinephrine system, while Dextroamphetamine acts primarily on dopamine, being 3-4 times as strong as Levoamphetamine in that sense (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2670101/). As a result, many patients find levoamphetamine to cause much more physical side effects like racing heart, jitteriness, etc. and be less effective at promoting mental focus than Dextroamphetamine.

Adderall is still 75% Dextroamphetamine, but I haven't seen much literature on why it would be preferred over drugs like Vyvanse that include no Levoamphetamine.