Why aren't there more knuckleballers?

Economic theory posits that in the long-run no company can earn a profit in a competitive environment, because somebody else will come in and copy them. But Tim Wakefield has been a fairly successful pitcher for the Red Sox as a knuckeballer for the past 6 years, eating innings and more importantly earning a major league salary at the age of 41. He has beaten or tied the league average in adjusted ERA for the last eight years. But, there aren't many knuckleballers: besides Wakefield, only R. A. Dickey (who plays sparingly for Texas) appears to be currently pitching with this style. If there are any parents out their who want their kids to play in the pros, I have the following advice: teach them to kick footballs, help them to grow an afro, or tell them to throw a knuckleball.