Why does Incorporator now want you to have a username/PIN?

So that Incorporator can save your information on your current run through Incorporator for your possible future use and keep it secure and only available to yourself via the entry of your username/PIN combination.

Why does Incorporator want your username to be your email address?

First because it is a unique identifier - there can be no duplication of a valid email address.
Secondly, if you lose or forget your PIN, Incorporator will only be able to refresh your knowledge of it, if you now choose to enter your correct email address. Incorporator will only communicate to you your forgotten username and PIN via the email address you now specify. This is a security measure.