21st birthday question?

I have so far invited like 20 people for my 21st birthday in vegas. So far five people have confirmed they are going, these include two couples and a friend of mine who's bringing another friend of his I don't know. The other 15 other people i've invited haven't said anything yet. I gave out the invitation like last week. I use a facebook invitation. Why are only couples going to my birthday so far? Is it just a coincidence or is it something about them? Why haven't the others said anything yet? I don't think it's money issue, my parents are even paying for a suite for them.