Yep, I think that's a large part of it, as the folks funding GitHub are quite open about the fact that they consider centralised US corporate control of the technology industry something to be admired, rather than deplored (See http://techcrunch.com/2014/02/13/please-dont-tell-me-you-want-to-be-the-next... for one of the most explicit examples of that genre). GitLab's business model is different, as it's just a low cost competitor in the self-hosted VCS market that takes advantage of people's familiarity with GitHub's UI, rather than aiming to become the de facto standard for open collaboration infrastructure (with corresponding influence over the career prospects of individual developers).
The free software movement has been fighting an underdog battle against that kind of centralised control for 30 years. That hasn't been a stellar success so far, with the likes of Amazon, Google, Facebook, Apple and Microsoft v2.0 now making their predecessors like IBM, Oracle and Microsoft v1.0 look like rank amateurs when it comes to exerting centralised control over the world's computing infrastructure. (Tom Watson's apocryphal prediction of a world market for maybe 5 computers seems likely to come true, it's just that their names will be AWS, Azure, GCE, Aliyun, and SoftLayer).
That doesn't explain why folks might be reluctant to state a preference for a proprietary service in public, though. For that, I think we need to account for the fact that the free software movement is often it's own worst enemy, as it tends to be *very* focused on ideological purity, so proprietary dependencies are considered categorically unacceptable in many circles, rather than as risks to be mitigated through pragmatic measures.
Trying to explain "We looked the gift horse in the mouth, checked all its teeth, and are happy we can deal with the risks of accepting the gift" can be incredibly draining when you have folks yelling at you that accepting gratis contributions of proprietary software and services mean you don't care about the future of the open source community or about software freedom. I stepped over that line myself back when the GitHub proposal was first put forward, and Guido quite rightly called me on it - I wasn't properly separating my own long term objectives from the immediate interests of the CPython core development community.
Since the pro-GitHub perspective was being suitably represented already (and was clearly the default choice, with most of the discussions focusing on "Is there a reason to *not* just use GitHub?"), why *would* anyone want to risk exposing themselves to that kind of potential negative response?