The problem with the Sharingan's higher powers is that the eye itself gets taxed and loses its sight. If there are no more Uechiha eyes to transplant, the best option is to preserve the eyes and to avoid using crazy overpowered jutsu like the Susanoo. Also, against high level ninjas who are nimble, a slow moving titan is not very useful as can be seen at the end of Shippuden when Naruto enters Sage mode and can easily evade big attacks. I'm sure as a father, Sasuke would have instructed Sarada to limit her use of the mangekyo, knowing that it will exhaust her sight and burn her eyes out. Likewise himself, he would restrict the use to only when absolutely necessary.
