Well, demand unfortunately. Other things are signed pieces, carrying a copyright symbol, makers tags, original packaging and the like. There are famous maker names like Trifari, BSK, Weiss, Eisenberg etc. Even more recent names like Monet, Sarah Coventry, Avon etc. are all collectible. Just because the piece isn't signed doesn't mean it isn't worth collecting.
Some people collect by theme.
Holiday themes, animals, bees, butterflies, florals, crystals, rhinestones, bakelites, geometrics etc. Collections of this form are particularly fun to display. I like to group my collection by the color of the rhinestones, crystals and beads. I just love it. When looking at a specific piece of jewelry, be sure and check for possible problems.