Well, I was wondering that same thing aswell! Bandai is probably only showing the characters that are new to the Tamagotchi. Or maybe they didn't want to make all 50 or so characters, so they made the most important ones!
I have also noticed that! Also some of the characters are named wrong which confuses people about which is which. If you are confused and only saw the tamagotchi.com 's character chart, visit the link Rey Mysterio put in his post.