Limited availability. Except for white and black, availability of size 20 long-prong snap colors will vary as new colors are added and older colors sell out. Please consider stocking up if you see a color you like as it may not be available again for some time after the current stock sells out.
By definition, long prong refers only to caps (the only pieces with a prong). Sockets and studs are the same regardless of the length of the caps.
Standard size 20 cap prongs are 5.6mm in length and are sufficient for most projects, including most diapers. Long size 20 cap prongs are 6.2mm in length and should only be used on very thick material, such as chinese prefold diapers. If you attempt to use caps which are too long for your project, then your snaps won't close properly.