The best payouts from signature were at 2015, where most signatures were paying 0,10btc weekly, and a few people here were getting 0,15-0,20 weekly with a signature without limit posting. Nowadays signatures arent enought to reach such ammount. If you have some capital, you can start your dice, casino or even exchange, those are the only places where you will be able to get some decent income over crypto world.
Either the price at that time per bitcoin was really low compared to how much bitcoins cost right now or there's just too much demand and less supply of users here or both. The price per post in signature campaign are based on its value in fiat. Just like in the Betcoin.ag campaign and secondstrade, they change the payment rate whenever there's a significant change in the price of bitcoin.
Then for my second point, there might be a lot less users here back then. Since it was just 2015, there's not much users here yet since bitcoin is just starting to get attention today. So the campaign owners had to compete to get more users participating under them.