I try to upgrade and found it is not free why?

You want more than you need, you pay for it.
Pretty simple.

Dude, have you seen how many other dating sites make you pay even just for using their forums? Or to email? It's ridiculous. You shouldn't have to pay for love, or whatever others wanna call it. And it's the big popular dating sites that have these cunning payment schemes. It's ironic, seeing as many people these days can barely afford bills, rent, food, etc. I won't be surprised if they lose more members than they started with. Money is for survival, not to spend on "dating" sites. They expect the public to suddenly cough up money for their website. Well i'm not silly enough to.

Welcome to the forums.
The site is free to use. The premium membership has some added features, but it is not needed to post in the forums, send emails, or instant messages, or even use the mutual match system. You can still use the site at no cost. Mingle2 has always been free, premium membership is optional.