The thing is, those other "arts" you mentioned aren't as unknown as you seem to think they are. The reason the arts like BJJ and Muay Thai (and especially wrestling) are so prevalent in MMA is because they are the ones that work best in a real fighting scenario. These guys have been training their entire lives and they don't overlook anything. Most professional fighters DO know how to deal with attacks from other arts because they train for it, but most professional fighters also know that most of them never work to begin with, so they don't even try. They stick with what works, which is wrestling, boxing, kickboxing, jiu-jitsu, and muay thai.
For someone that's only ever trained in one or two arts, of course they'll be surprised with things in an MMA fight. They also won't last long in MMA.