Emphasis mine; it isn't.
The Big Bang isn't a gigantic explosion at a single point in space and time, but rather the creation of an entire space-time. The space created is either infinite, or finite but without any edge (curved back on itself). In either case, an observer at any point P in that Universe's space, and at any time T in that Universe's history, can intercept exactly and exclusively those Big Bang photons that were created at any point T light-years* away and that happened to be launched towards P at time zero**.
*Actually it isn't quite T light-years because of the dynamics of that Universe; rather, it is the distance photons have traveled in the age of that Universe.
**Actually it isn't quite time zero, but the time at which that Universe first becomes transparent to photons, which also depends on the dynamics (expansion => cooling).