Terms of the acquisition, most likely. Existing customers are worth $x/yr. New customers with the new product are worth $y/yr. Finance doesn't want $x competing with $y. Something something financial models making the deal worth what they paid, so closing registration is simply part of the deal that was signed.

They want to move the customers to a new AWS service (that will be this product rebranded with AWS) and tie into AWS infrastructure instead of using a special system for sign in.

By preventing sign ups, they fix the number of accounts that will need to be migrated in future.