Robert Jordan said that you could not reach Shayol Ghul using the World of Dreams. When characters try to do this, there is a barrier preventing them from progressing. This implies that the Dark One created this barrier because It is especially vulnerable to attack there.

Just because you can't easily reach Shayol Ghul doesn't mean you can't enter Tel'Aran'Rhiod once you reach it.

A good example is the barrier that the Dreamspikes' created in Tel'Aran'Rhiod - in many ways, they resemble the dimensional disturbance surrounding the Blight and Shayol Ghul preventing easy access via T'A'R and gateways. Even more noteworthy is the fact that Moridin was stockpiling tools that specifically prevents Travelling via Gateway or Skimming. Mesaana also started stockpiling specific ter'angreal - it is worth remembering that the strongest angreal the Tower had were left alone, but she took the opportunity to steal any ter'angreal that was connected to T'A'R as soon as possible. This implies that Tel'Aran'Rhiod and Travelling are important to the Shadows' strategy, and I think this is because the Bore is especially vulnerable to attack from T'A'R which may be why there has always been a defense from having their territory penetrated via the Dreamworld. We already know that even if Lews Therin wasn't completely successful, he was able to attack the Bore with an effective weapon, and it seems to me that the Shadow is making it as difficult as possible for this to happen again.

I think this is why Rand and Lews Therin are specifically stated to have a Talent for Travelling, even though we haven't really seen what this Talent would consist of besides blocking them from forming. I think he will need to find a way to Travel close enough there to attack the Bore directly or access the part of the Bore that exists in all dimensions and realities, which would bypass the need to physically travel to SG in the first place.