Fans forums are popular because anyone can immediately access them and contribute. Location doesn't matter either. The internet allows you to get information anywhere. Being in the midlands, france, madagascar... whatever. Means nothing, it's all here.
It is the immediate medium, you want info, it's here, you want speculation, you can start it, you want crap.... well.... you can read what you want into anything.
Saintsweb is popular because, as implied by Saint Andy 666, it is the direct continuation of Saintsforever via the Saints Forum. They were popular and have continued. There are others (TUI for example) but this site is the biggest and best due to it's lineage. What do I like? It's interactivity. You genuinely do get out of these what you put into them. I am now a site contributor.... that would never happen in anything more official, something needed doing, I wanted to help and here we are. If anyone cares enough they can make something happen. I'm sure you guys at the Fanzine feel the same.
This age of communication is great, isn't it.