I think because in the story itself, Ciel and Ichika are overlapping too much in term of character. Yukari-Akira and Aoi-Himari are opposite of one another (different character type, different interest, etc.) and in this Precure the paired one seems to be those that contrasting one another. Ichika and Ciel on the other hand has the same interest, both are very confident and outgoing, both has family problem, etc. and even things that different between them aren't significant enough to make them a contrast.
From what I see, Ciel-Bibury has bigger chance to be paired than Ciel-Ichika. I think even the twin don't ship Ciel-Ichika in the manga (they still ship Ichika-Akira).
Merchandise don't have this problem (as they usually just a pose without context), so it is much easier to do.