Why are the username and password on two different pages?

The most common reason to put username and password on two different pages is to support both:

However, this login flow confuses people which is probably why you’re reading this! This post looks at the security of this design decision and presents options for designing login forms that support multiple paths of authentication. Presenting the fields on a single page (Option 2 or 3) also allows additional authentication options like adding social login buttons.

Source: www.twilio.com