Why does her visa got declined and how do I get her to the US?

You need to find out first the factual basis for the Consular officer's denial decision. Ask her to send you the visa refusal work sheet she was provided after the denial. The refusal work sheet will include the legal basis for the denial. And unless the refusal was with prejudice, she can re apply again if she can produce documentation to overcome the grounds of denial.

Visitor's visas are hard to get especially for someone who fits the profile of having immigrant intent. You can try to have her apply for the visa again with more documentation to try to overcome the immigrant intent.

Of course not! She is a single girl coming for a visit to the US! The problem is that the consulate would perceive her as someone who would come in as a tourist, but never return home. Especially since she has a special someone she wants to go meet in the US, of course, the consulate would deny her a visa. So the best thing you can do is go to see her in her home country or some other country where you two can meet. And then, if things get serious, you can apply for her fiance visa - but only after you two actually meet in person. FB and Skype are insufficient. I am probably looking far ahead into your situation, but the bottom line right now is that this denial has nothing to do with your level of income and your obvious ability to provide for her financially while she is here. The gist of the problem is that this is a single lady who may never leave the US (in the eyes of the US consulate officials). There is nothing you can do to appeal this decision. She can certainly try to reapply, but she would need to show strong ties to her home country, a job, ownership of property, family members, such as a kid whom she is leaving behind while traveling - and even all that is no guarantee that a tourist visa will be issued. Sorry.