I believe that the current consensus is that there is a 15 LY range in place to tie new systems into the BGS and PP. To even hint at removing either BGS or PP and it is likely to meet strong resistance from those that play those parts of the game.
This is what a lot of players seem to have decided on, but it's almost certainly not true, or at least not a major part of the reason.
On the BGS side, there's no BGS mechanism with a 15 LY range limit (there's a couple of 10s, and a bunch in the 20-40 range). Almost all of the issues (mainly mission board ones) which would arise with a genuinely isolated system would stop the moment there were a pair of nearby systems. All BGS activity is localised; there's no requirement for or checking of longer-range connectivity, and most BGS mechanisms work fine even in completely isolated systems, of which Frontier has added many. And colonisation is breaking all sorts of previous BGS behaviour and true-most-of-the-time assumptions, too; who
cares if a new system is within BGS expansion range for a faction now that this is no longer the important way that factions spread to new territory?
On the Powerplay side, the pace of Powerplay expansion is so slow (and it's not supposed to even be that fast, because the Powers are
supposed to be attacking each other) that most of the new colonies not already within the existing Powerplay spheres will
never be touched by it. My own colony is I think just two "Powerplay jumps" from the edge of Kaine space with hundreds of possible routes and - unless I happen to decide to do it myself at some point - I think there's a fair chance it will remain neutral for the next decade. Powerplay has a maximum long-term progression rate, regardless of the amount of effort put in, of 10 LY a week. Some of the faster long distance chains are going at well over 200 LY a week.
Some alternative reasons, all of which might also be false, but which are I think more plausible:
1) It means that if Frontier had messed something up seriously with the Beta, the maximum damage would be limited. This implies that it might be increased later, but then, it might not.
2) It discourages systems getting far enough out that they start to have bad interactions with the lack of local detention centres (some of the longer chains are already past the sensible distance for this even with a 15 LY range)
3) It's deliberate to make it so that long range colonisation projects are epic and memorable group events (of the sort that some of the pre-release Exploration communities in 2014 thought long range exploration might be), not something where several thousand players complete their own individual ones in a week or two.