Hi Farzin,
we only include the WebServices that we need in the Mobile app. This is because some WebServices don't make sense for the Mobile app, and for security reasons (the less available WebServices, the less actions an attacker can do if he gets your token).
If you want to add more WebServices to the MM service, you can do it in your site by adding this in the WebService:
https://github.com/moodle/moodle/blob/master/mod/lesson/db/services.php#L45
Also, if you have a plugin that includes new WebServices they can be added to the MM service too.
Cheers,
Dani