According to Keith Olbermann, a far-ish left progressive, on his podcast The Late Show's cancellation it really was primarily due to financial difficulties of it being too expensive in a declining industry of traditional media where there are barely any OTA TV or cable viewers. Their overhead is too much and their internet audiences don't make them enough money. This is a classic horse and buggy company failing to adapt to an automobile world.
He also made it known he doesn't care personally for Colbert who rapidly took a proverbial wrecking ball to David Letterman's set to effectively damnatio memoriae his predecessor and obliterate all potential memorabilia.