There are few reasons for that.
1. since iOS5 emoji keyboard layout is available for selecting by default, there is no need in third-party apps to unhide;
2. there are too many duplicate apps;
3. some of third-party emoji activators are not free;
4. some of third-party emoji activators send user personal information to unknown sources (most of people who would like to unhide emojis will ignore and confirm privacy alerts)
I understand what you are saying, however you might misunderstand my question.
I am not talking about the unhide function of those apps, but the reason why Apple wants to hide some of them. There are some emojis such as dragon, ladybug, ant, crocodile, and camel. Those emojis exist in iOS but you cannot see them in the default emoji keyboard. There are some apps call those emojis "Emoji 2" and they can help users unhide them.
I think Apple might hide them for some unknown purpose.
Thanks for your reply!