ARIGHT theory time :D
So I believe The Backrooms in it's entirety is 'connected' to the reality of The Frontrooms but there's a significant gap between the two & with the two realities being so close together sometimes vacuums form that bridge the two together, from which people can fall into (noclip) and enter The Backrooms.
Now the Backrooms itself very obviously isn't just a reality, it has some amount of sentience. So now with that in mind it's most likely aware of this gap, and with it's mindset try to fill this gap. Now enter the blue channel; with the BRs vastly different structure of reality it can't fully combine with the Frontrooms without catastrophe, so naturally our reality will try combat this. Once in combat the blue channel takes form, the reason it does is because the BR tries to fuse with our reality by taking chunks of it through the already formed vacuums.
Evetually the blue channel takes form and acts as a sort of bridge between these two realities. Now why do we not fall straight into here when we noclip? Because the elements used to form it came from polar opposite forces so it nullifed the outcome to create an area that cancels out the majority of laws the BR has, therefore making it not 'reality', while FR elements make it 'human'. This means the vacuums created beforehand pretty much ignore this area because it isn't it's own reality so it doesn't give vacuums a sufficient anchor without completely breaking them; so noclipping remains the same. The BR can't do anything without drastically changing form since laws of reality can't actually differenciate the BRs from the Blue Channel so it can't properly detatch unlike the FR as it can easily separate itself from the Blue Channel. This explains why you don't initially end up there but fully and well can.
TL;DR: the Blue Channel acts as a bridge between the Backrooms and our reality (Frontrooms) because the BR tried to fill a gap between itself and us. The gap filling caused a natural defence mech and had the FR fight back, creating the blue Channel by clashing.