splitting the login process into two screens (username and password in two different steps)?

Question

Why are more and more portals splitting the login process into two screens (username and password in two different steps)?

Answer 1

This is not always the case. There are some other that you enter your mail, then go to your mailbox and by clicking you enter, no password is needed. (Medium – Where good ideas find you.)

But, this is the simulations of Mainframe’s login is a historical process from the first computers with keyboard.