Board Thread:Manga/@comment-27821571-20160923192030/@comment-27441457-20160923195813

The Coordinate can't be used properly by anyone outside of the Reiss bloodline. Eren's use of it in Chapter 50 is the closest any non-Reiss has come to using it, and at the time his mind was in a frenzy.

Grisha isn't a Reiss and he was never in any situations like Eren where part of it would leak out, so he couldn't use it at all unless he knew of a way to use it by getting around the Reiss lock on its power.