Scientists probe the mystery of Titan's missing deltas

On Earth, deltas form when rivers slow down as they reach larger bodies of water. These formations are important to scientists because they preserve geological, climatic, and even biological history over long periods.

So the article is saying that Titan’s lack of deltas is surprising because of its active river systems of methane and ethane.

Means that maybe we're wrong to assume that wherever there are flowing rivers of liquid and sediment, deltas will naturally form.