More like running out of money.
Cameo is a site that's just now picking up steam. They're adding more names week by week, some of them are pretty big. You also have to keep in mind the most popular names are also the most expensive(Lena Headey/Cersei Lannister is at $300+) and I just don't have the money right now to toss at those folks.
I do tend to shoot for more well known names, but celeb status doesn't really matter much to me. I just like hearing people admit their ticklish. I've also become fans of celebs, who I had no idea existed before this thing(like Jade Tailor).
If anything, people can use this thread as a database to check back on in case any one of these lesser celebs happen to blow up in the next few years. People can check to see if they had been confirmed.