Why does iPhone Safari not save login credentials for a certain Wi-Fi hotspot?
check out this link http://www.ausbt.com.au/how-to-save-passwords-in-safari-on-the-ipad-iphone to setup autofill name and passwords for safari on IOS.

And it totally is a matter of safari not remembering username/password.

It's a little hidden, but it's there, and thanks for asking a good question. The other answerers here must never have logged into a captive portal before or something.