High-achievers are skeptical of free money.

They believe that you get what you pay for, and that their time and attention is valuable. (It's a defense mechanism that helps them believe their achievements aren't attributable to sheer luck.)

So high-achievers are a lot less likely to click on a web link that says “click here for $10” than they are on a link that says “click here to sign up for an expert mailing list which will take time and attention from you but may yield you $10 if you are a top contributor this month”. Even if the first link really will earn them a quick ten bucks.