'Outdoors' and 'cat6' cable is where the problems start - first you have the separator in cat6 that makes the cable bulkier, then they seem to have the different insulation you mention above, then they can be gel filled as well, then the sheath is different (and potentially doubled).
The problem is usually the cores get stuck on the way into the RJ45, so you can't fill a whole RJ45 at once. With passthrough you can manipulate them by pulling at the far end, which is much easier for getting everything in place.
Theo