The other problem with AC that's making the value of bells deflate is that people really don't need to buy much anymore. Need a PWP or some Gracie furniture? A trip or two the Island provides everything you'll need for that.
And most items are reorderables, so even if people want DLC's or to catalogue sets/other items, they can easily find some for rather cheap from others. Holiday items? Time travel. Villager pics? Also time travel. Even taking duping out of the equation, with older players with 100% completed catalogs and players that are willing to TT for others, it isn't too expensive to buy the items needed.
That just leaves villagers. Most people probably sell as many as they buy, and once people hit their 10 dreamies, a great deal stop.
So even just legit users accumulate tons of bells that they have no purpose for. BTB, on the other hand, can buy collectibles and add-ons for the site. So it makes sense that people who are finishing up with their goals on ACNL turn their focus to BTB instead.