Different weather. It is doable if you have 2-3 weeks. If not, just concentrate on BA and Iguazu. Plan 5 days for BA and 2-3 days for Iguazu.
You will enjoy it more if you plan on doing less. If you really want Patagonia, looke at 3 days in Patagonia, 3 days in BA and 2-3 days at Iguazu