After four months exclusive to comiXology, Marvel's Immortal Iron Fists by Kaare Andrews and Afu Chan is out now as a print collection and on the Marvel Unlimited service.
Originally announced in July 2016 as a print series, Immortal Iron Fists was shifted in 2017 to be the first of several Marvel titles exclusive to comiXology under the "comiXology Originals" banner (which other companies such as BOOM! Studios and Valiant Entertainment) have done. Since then, Marvel have released several other miniseries exclusive to comiXology such as Thor vs. Hulk: Champions of the Universe and Black Panther: Long Live The King.
While the window of exclusivity to comiXology is unknown, there was a four month gap between the final issue of Immortal Iron Fists and its release as a TPB.
This is similiar to the handling of Marvel's live-action Netflix series, which are exclusive to that streaming service for a time but are later released by Marvel independently - in those cases, on home video.