Why are Colin and Eloise unmarried during so much of When He Was Wicked?

Part II of When He Was Wicked (Book 6 in the Bridgerton series) takes place concurrently with Romancing Mr. Bridgerton (Book 4) and To Sir Phillip, With Love (Book 5). Therefore, even though When He Was Wicked comes later in the series, it jumps back a little in time at the beginning.