There have been a number of concessions over IGTV in the past. Instagram eventually allowed landscape videos rather than insisting on vertical clips, and it just recently enabled the option to post IGTV videos directly from the standard app. This, however, may be one of the more significant changes yet -- it's taking away convenient access to the feature.
The spokesperson didn't say why use of the button was low. It might be a simple matter of the way people use Instagram, though. As a social network, people primarily use it to catch up with friends and share snippets of their daily lives -- they're not necessarily going to watch a five-minute professionally shot video they could probably find on YouTube. Add in the lack of revenue for creators and there aren't many reasons to use IGTV when the usual shorter Instagram videos will do the trick.