I think they are so popular because there aren't many alternatives, but once there's more professional advertisers in the world of cryptocurrencies, the role of signatures will start to decline.
This is where bitcoin user resides. There are big players out there but they are not in most forums or are too busy to check forums so they just lurk on Telegram and Slack. Campaigns look expensive on our side, because we're just regular guys. But for someone that has his own company, it's not that much especially to those gambling site owners. If you've been in a gambling site long enough, you'd see people having more than 1BTC losses and there's tons of them. Signature campaign payments are just a small portion of their profits so they don't mind keeping it alive even when it's not as effective as you'd expect it would be.
On a side note, I believe that some companies keep their signature campaign in here just so people will remember their company's name. Most campaigns that ended their signature campaign a long time ago are already forgotten by now.