I am on several ranked fives teams and don't recall having to complete any series. Not sure if it is because our MMR is high or because series are non-existent in ranked teams.
It is actually because of the MMR.
You will realize when playing the placements of your ranked team and loose many of them and get ranked badly, but then win every game you will skip some promotions. But the higher you climb the more you loose (mostly) and you will stop skipping the promos.
Obviously there are no tools to calculate your team MMR officially but it is definitely based on the soloQ rankings.