Because the flying public in the United States expects a jet when they buy a ticket.
Also because FedEx is not a passenger airline.
Look up Roselawn IN accident. After that happened American Eagle moved all their ATR’s to warm climates to avoid icing issues. Around 2005 they phased them out completely. David B. is correct, American airline passengers do not like to ride on anything with a prop. I have seen some cases in the pt135 world where a charter business was lost when the client figured out they were paying for a prop plane. Even the Q400 is not flown any more in the lower 48.
That is a pity, as the 400 fills a sweet point for many routes, the ATR has a problem with S/E performance. What is this magical jet that carries 36 people 400 miles?