Welcome to Android Central! The SD card has always been used primarily for media storage (photos, videos, music), not as a way to install more apps. Not all devices have a "Move to SD" option, and for those that do, they can only move a portion of the app to SD. Newer phones on Marshmallow may have an option to format an SD card as part of the Internal Storage (which does increase the phone's app storage capacity), but it's not perfect: Inside Marshmallow: Adoptable storage | Android Central