A very short answer is because Eren doesn't know how to.
Eren has what that has been called as "The Co-ordinate" which was passed to him from Freida Reiss through his father. This is the ability of the Founding Titan which allowed them control over the titans and memory manipulation.
We do not yet know how exactly this power works. The real limit of this power is unknown, but as it can be observed, it extends to the point of making Titans perform actions that threaten their own lives. It was also mentioned by Rod Reiss that the power is so strong, it has the potential to wipe out all the Titans if the user is able to wield it to its full strength.
Then why it was Eren was unable to use it while he had contact with a royal bloodline, the reason is that Eren has only shown this power under very rare and unique circumstances. One can argue that the danger to his life was one such circumstance, but unless the writer clarfies the exact nature of this power we don't know the limitations.
For more information, with HEAVY SPOILERS refer to : Wikia : Founding Titan