Board Thread:DC's Legends of Tomorrow discussions/@comment-9346948-20160413000408/@comment-3423279-20160420011522

Probably because Thawne was very careful about not disrupting the timeline and Nora Allen wasn't nearly as important to history as Vandal Savage is.

Plus.. the Time Wraiths seem to be tailored to deal with Speedsters that disrupt the timeline, and if the Time Masters know about them they probably let the deal with it.