The UK's existence came about with the union of its constituent parts - a family of sorts.
If three members of a family of four adults (say people from the UK, living in the UK) decide they are going to live in Australia but the fourth does not want that, it would not be reasonable to say that the emigration must go ahead and apply to the entire family. Once the three leave, the fourth is still eligible to remain in the UK. Now replace UK with EU.
As has been said above, last time the promises persuaded the wavering but that has gone sour. Similarly, the scare tactics have been worn out and the suggestion that an independent Scotland would not (if it wants to) continue more or less seamlessly to be a member of both the EU and NATO is now taken seriously by far fewer than before. A referendum on independence held now is much more likely to result in a Yes - after all, it would only take just over 5% to reverse their position and I have not come across any suggestions that those who voted Yes last time are changing their minds in significant numbers, on the contrary.