You'd think more start-ups/mid-size ones would use Bitbucket since they have free private repos. I'm guessing the social allure of Github and its superior repo/pull request UI is what trumps Bitbucket to that end.
I'm pretty sure it's wanting to be one of the 'cool kids' that pulls people to GitHub.
If your decision about where to host your code repos is decided by how pretty the webpage to create pull-requests is, the wrong people are making decisions such in your business.
The differences aren't as superficial as you suggest, in my experience. GitHub is just far more polished. Take commit history as an example: bitbucket makes it uncommonly awkward to step through a series of commit diffs for a given file, whilst github makes it a bit easier (although still not perfect - am I the only one who wants this feature?). Nicer looking doesn't always mean more usable, but there's often a correlation and, in this case, it definitely bears out.
Having said all that, I'm currently using bitbucket for a private work repo because I'm cheap :)
Unsurprisingly, this is very much a personal choice. I vastly prefer BitBucket's issue tracker. I love their side-by-side diff (somewhat recently added to GitHub). I love that they don't try to force this 72 char commit message on me and seem to handle it rather nicely via hover text. And I very much liked that they didn't support emoji and inline GIFs (since implemented, lamentably).
The more GitHub seems to adjust itself, the less I enjoy it. The more BitBucket seems to try to copy GitHub, the less I enjoy it. At the end of the day, I probably want GitHub circa 2008. I find when they really became opinionated about things to be the inflection point about whether they pushed things that were truly more usable.
That's not to say your view of things is wrong for you. But I don't think it's clear that "in this case, it definitely bears out".
> bitbucket makes it uncommonly awkward to step through a series of commit diffs for a given file, whilst github makes it a bit easier (although still not perfect - am I the only one who wants this feature?)
I want it too :) A time-machine style forward & back, showing changes to a file.
