The login method employed by flipkart is vulnerable to MITM attacks.
The form/page is not served over HTTPS, thus, content can be injected into the page by anyone between yourself and flipkart. Not as secure as it probably should be. (see details on this blog post by Troy Hunt).
Our login forms are always served over HTTPS. Making them popups will only make them less secure when they come up on pages served as HTTP.
Arguably, we don't deal with money on the Stack Exchange Q&A sites, but we do take username/password pair for the Stack Exchange OpenId provider, that can be used anywhere an OpenId is accepted - so making the form less secure is not an option.
This is a minor annoyance to you, but imagine the annoyance if your username and password were harvested by a malicious third party and used for nefarious deeds.