I think part of the reason may be that, by the standards of beads, a lot of regular beans aren't really durable enough for long term wear. A lot of the legumes on that list have the kinds of seedcoats you need a pair of pliers to crack. The one time I tried to grow Caesalpinia bonduc I needed to use a grinding wheel to breach the seedcoat and hold the seed to it by hand, a process I REALLY don't recommend. Nickernuts are REALLY good at holding heat so they get red hot really easily under friction and you have to sand off ALL of the outer seed coat for water to get in evenly (they aren't like most of the hard legumes, where, once you make a hole in the outer coat for the water to get in, the rest of the outer seed coat softens up or flakes away as the under coat swells so that the seed ends up imbibed from one end to the other). Compared to those, the common bean is a little brittle. I suppose it works okay for more ephemeral pieces, but a lot of these jewelry items are heirlooms of great spiritual significance to the people who make them; designed to be worn for years and passed down.
Oh and speaking of nickernuts, a clarification from what the article said. Yes "nicker" is an old word for marble but the seeds were never used as marbles in the sense we would think, for the game of marbles. They can't be, the aren't round, and actually it's pretty rare to find one that doesn't have some flattened sides from pressing up against it's neighbors in the pod (you CAN (I have one) but it's like trying to find a perfectly spherical chestnut), so they don't roll. You can use them in a game sort of like tiddlywinks but their most common game use is for something called Island Wari which is a simplified form of mancala (or I suppose backgammon) Supposedly it was invented by the Caribbean pirates. You make a series of holes and each player gets seeds (traditionally, one will use the grey nickernuts, and one will use the related Tan nickernut (C. major/christa).Then it's the standard move your seeds along the circles until you get to the big circle at the end type game)