Because we have had 40 years tied to this EU behemoth, means untangling it will take a great deal of time. Simply repealing the ECA 1972 hoping it's 'a giant red reset button' is completely unrealistic:
Scotland is not going to vote to leave the UK, and then become independent the next day. Disentangling everything from energy networks to transport systems, financing to contributions to the BBC is going to take a long period of time – at least 12 months. It is going to be a matter of enormous policymaking complexity. Now I know everyone in British politics assumes negotiations can conclude instantly (Nick Robinson’s 5 days that changed Britain, about 2010, is testimony to that), but I cannot see how separation of Scotland from the UK could possibly be concluded swiftly.
In comparison to that, Scotland applying to join the EU is actually going to be comparatively easy, and most definitely much easier and faster than any previous enlargement of the EU because Scotland is fully compliant with the acquis communautaire anyway. Hence how Scotland can work within the EU can be negotiated in parallel to negotiations with London to leave the UK.
Now there is the small chance that something could go wrong – some country or other could veto Scotland’s entry. But doing so, for a comparatively rich new country that had been part of the EU anyway, is just going to look like sour grapes and anyway some major EU countries, notably France, will be content to see a weakened London anyway, and hence would be on the side of letting Scotland into the EU. Yes, Scotland might have to commit to join the Euro, but Sweden still has that commitment as well, and is it making it happen?
Also look at the UK-Scotland side – what happens if these negotiations were to fail? That a financial arrangement, or a division of military or natural resources cannot be hammered out? Again, this looks to me much more of a headache than an EU accession.Scotland becoming fully independent does not happen in practical terms by merely repealing the Acts of Union, thus neither does the same happen to us repealing the ECA 1972 in regard to the EU.