This forum wasn't made to give someone any earning opportunity when it started. It was in the forum.bitcoin.org domain, and the purpose was to discuss Bitcoin. Later Bitcoin.org admin Sirius requested theymos to move this forum to another host even though theymos did not want to move. Later theymos moved the forum to the current domain, and it's still running here.
It changed with time because signature campaign is not in the forum in 2009 and early years. A first signature campaign is in 2013 and it is a big change. However it can be changed again in future as in 2017 and 2018, theymos seriously thought of mute signature and signature campaign if merit system fails.
History - signature campaigns and bounties on BitcointalkEvolution of signature ad campaigns (beginning days to the present)