I look at it in a number of ways, as to why most boyscout troops primarily carry a folder, even though a fixed blade isn't forbidden.
Firstly, I believe that tradition plays a big role, they don't call them "scout pattern" knives, for no reason. They've been an integral part of boyscout history for as long as I can remember, and most likely long before.
Second, I believe that folding knives help to teach young boys how to better handle a knife and what is and isn't acceptable. For instance, it's not a prybar, you don't cut with the spine, it's not a throwing knife, and it's not an axe or saw. Because there are certain "limits" as to what a folding knife can do, it's used just as it should be, as a small cutting tool and perhaps an expedient tool for other small tasks.
Lastly, I believe that the tradition of carrying a folding knife and it's popularity can be summed up in the BSA motto itself-- "Be prepared." By encouraging young boys to carry a pocket knife that can always be with you(unlike carrying a fixed blade which has more restrictions placed on it) we're encouraging good habits, instilling responsibility and self reliance at a young age. There's plenty more that can be said about this, but I'll stop there and just say...
There was a time when no man was worth his salt that didn't carry at least a few basic accoutrements--A pocket knife, a watch, a handkerchief, etc.
The sad reality is, that the BSA are one of the few organizations still encouraging such sensible behavior, despite it sometimes going against the grain of popular or "politically correct" opinions.
In today's social climate, unfortunately you have to pick your battles sometimes, and I think the BSA has made a good decision in advocating
the use and carry of pocket knives. I believe a lot of it is image consciousness in order to avoid more stringent restrictions. Could you imagine if certain people had their way? A boyscout may not be able to carry a knife at all. An 8 or 9 year old carrying a fixed blade is perfectly normal, the times we live in however, aren't.
Gautier