So you don't know. You could have just said so.
Let me explain: Of the three synoptic Gospels Mark is the most condensed. Luke and Mathew both contain all the narratives of Mark with wording even similar in places. So Mark is viewed as primary source material because it is a simpler narrative and is found in both Luke and Mathew.
Source Q is supposedly a source for Luke and Mathew because both share material. However because there are differences within the same narratives, there might be other sources for Luke called L and M for Mathew.
If you notice the method of determining chronology is quite simple: Simpler gospels are reasonably dated earlier than later more complex gospels. And that is the primary way that the Gospels are chronologically placed.
So why can Thomas as Q be considered a primary source for Luke and Mathew. It does not contain narratives but just sayings of Jesus and thus can be considered the simplest of all available gospels. Hence according to the method employed with Mathew, Mark and Luke the earliest.
Another point that is salient is that Thomas as document is very important because it indicates that very early on in Christian history sayings of Jesus on their own were being passed around within Christian society.
Why couldn't you explain this? It is very straight forward.