Cancun and Punta Cana are vastly different tourism markets. Its almost comparing Hawaii with a South Pacific destination like Fiji or Tahiti.
Cancun is a mass tourism market, with over 125 hotels in the area with some 35,000 rooms, of which 20,000 are in the immediate concentrated hotel zone alone. Punta Cana on the other hand is a much smaller tourism market with last I read had about ~7,000 hotel rooms, of which the majority are all-inclusive resorts itself something that is not as popular in the US as independent travel is.
Also Cancun is a much closer destination from the US to reach being across the Gulf which makes it attractive to even West Coast travelers. Mexican tourism authorities have done a fantastic job since the late 1970s building and marketing Cancun and Mayan Riviera making it today one of the world’s leading tourism destinations. Punta Cana is much more of a niche destination and one for the East Coasters for geography and for Europeans due to the all inclusive nature of many facilities.
From the desert to the sea, to all of Southern California