Why is Adderall (dextroamphetamine and racemic amphetamine) more commonly prescribed for add/adhd medication than the other drugs?

Cheap and effective. Generic Adderall (dextroamphetamine and racemic amphetamine) is much less expensive than other adhd drugs and for many people it works well and has fewer side effects.
Name recognition. Adderall and Concerta ruled the roost for many years in terms of most commonly prescribed medications for adhd. Vyvanse (lisdexamfetamine) is gaining ground these days. The two stimulant classes are amphetamine & methylphenidate, with each having different brands that vary in terms of length of action and delivery method. Doctors (& patients) develop favorites, but they work pretty similarly.
Often effective. Adderall (dextroamphetamine and racemic amphetamine) is often effective in treating ADHD (high effect size). It can be dosed in small increments and is generally well tolerated. Major concerns would be ongoing substance abuse concerns, heart or blood pressure concerns, existing psychosis, major troubles with sleep, concern for making anxiety worse, loss of appetite, wear off effects including irritability, diversion, etc. is generic.