We're going with a decade of evidence that this game was originally designed around a 20ly range and multiple incredibly significant pieces of content and even new game engines where this was not addressed. And it was not addressed in PP 2.0 either.
I don't know why they chose 10 specifically. I do know why they chose a number under 20 specifically.
It's okay, wait till people realise, that if they don't start on the very edge of the bubble, no matter how hard they try, they won't be able to escape it.
Unless frontier supports multiple different locations and not just 'the bubble', commanders who start closer to the centre are going to find they can't expand past people who started at the fringes (which are almost certainly going to be the most popular place to start).
Being stuck in the bubble for an indeterminate period. Potentially years. How exciting.
The only thing that probably helps is that I can't imagine too many are going to bother with more than one system, let alone hundreds. So there's that, I guess.
Also, at 20 ly, colonisation can only go so far out, before distances between stars prevent further expansion. That happens much sooner at 10 ly. The bubble is already large enough the gaps between stars become noticeable.
Colonia has much better potential, ironically. Star density is excellent.