Mongolian Cyrillic for iPhone
There are no Apple solutions for adding keyboards or fonts, only third party. Search the app store for Mongolian and try the Keyman app.  Unfortunately you have to upgrade to iOS 8 to have a keyboard that operates for the entire OS and not just via copy/paste.

Nobody here can tell you why Apple hasn't included some feature you want, but you can ask for it via

http://www.apple.com/feedback