I think the difference is fairly obvious really. Scotland voted by a large majority to remainin the EU and as a result the SNP repeatedly declare that Scotland is being "dragged" out against its will. Theresa May has said that the UK will exit the single market and the SNP has said that this will be damaging to Scotland's economy (let's leave aside the fact that Scotland exports four times as much to the rest of the UK and so using their own argument, independence would be four times as damaging.) There will be "a hard Tory Brexit" as the SNP put it. But the aim of the Government in the negotiations is to try and secure a comprehensive free trade deal. So why not wait until after the end of the negotiations and two year Article 50 period and see if they have been successful? Then the Scottish electorate will be able to make a decision (should they want an independence referendum) based on that knowledge.
The independence referendum was different. The Leave side based their campaign on taking back control (which Teresa May is honouring by leaving the Single Market) and argued that the economy would be more successful outside the EU. The Remain side based their campaign on warnings of economic chaos (Project Fear), which so far have proved to be completely unfounded. But the fact is those warnings were made loud and clear before the referendum so the electorate had full knowledge of them but still voted to leave.