For me it is explained that way:
WBTB is a forced way of opening your “dream enclosure”. You can wake up and remember them, start thinking about it and realise when it happens again.
But the question “Why is it easier for most people?”
That’s just like i said, that forced way of remembering may work even when you cannot can induce thoughts to your mind.
Maybe caused of stress, bad memory, tiredness, bad concentration…
I’ve mentioned it here.