Are you assuming the poly people you meet aren't interested in more primary relationships because they already have one (or more), or is it something they stated themselves?
Or does it come from you? That is, do you specifically not want a primary relationship with someone if they already have a primary relationship with someone else?
Aside from that point, primary relationships require more investment and I can see them being more difficult. They're also about commitment, and not everyone wants to commit right away. Lots of primary relationships, poly or mono, didn't start that way, but evolved to that point.