The answer is simple: Fleet commonality dramatically reduces costs.

If there were, say, an A220-500 and A220-700 under development that could feasibly replace the A320 and A321 respectively, they might have considered it. As it stands, they have 400+ of the A320 series that can all be flown by the same pilot group, have common spares and maintenance needs, and can be shifted as needed. The A220 has close to zero in common with the A320 and, as such, requires a different type rating for pilots, has wholly different spares and maintenance needs, and would offer AA a lot less flexibility.

Despite my irrational love for the 737, it's a head-scratcher for me that AA even continues to have plans to operate them over the long term. The extra 20 pax over an A320 or the marginal cost of operating an A321 with 20 empty seats must be really compelling to them.