It might just be the most pick-up-and-play game on the site, and unlike many games here it's not "hindered" by either graphical or interface issues since the collection is so simple to begin with. Hell, in a lot of ways it's got more in common with PopCap's lineup than most everything else here. From that perspective, it's not at all surprising that it's doing well.

That's slightly different.
I'm talking about BUSINESS. GoG makes a business out of selling these games... But I do see where you're coming from, I just would be extremely surprised if they actually told everyone how much they're making all up. Do you get where I'm coming from?
It's slightly different IMO.

Im still a student : )

I always saw Fallout as a sign of how well new games were selling. It seemed to sell a steady amount each week, mainly (I/we think) to new visitors. So if Fallout was high on the top sellers list, the new releases were generally not very popular, as they failed to sell more than the steady and predictable amount of sales that went to Fallout each week.
But things have changed a bit. First of all, it's no longer exclusive to GOG, so sales numbers must have decreased as a result of that. I think this is somewhat good news, as it means GOG must try even harder to attract new customers. They can't rely on the old Interplay-classics. Second, we've had some very good promos lately. In general, discounts seem to become more and more important for all digital distribution sites.
Still, we have had some seriously good releases lately. TIM, TLJ, M&M, Arkania, HOMM... it's only natural that these are pushing Fallout and the other regulars (Freespace, Duke 3D) down a bit.
Super-late edit: I guess we can see the effect of Fallout being on Steam now - Freespace 2 was usually a position or two below Fallout on the top ten list. Unlike Fallout, it's not on Steam - but it is still where it used to be on the GOG top ten.