There are some differences between the two parties, to be fair. SNP also support the abolition of trident, which is a pretty big difference, and are moderately against any extra austerity, which Labour are reluctant to commit to.
Even if the two parties are largely the same though, what the SNP have been good at is convincing voters they do have a solid set of guided principles. Even if you’re sceptical yourself, they’ve worked very effectively as a party, and have managed themselves well. They’ve learned what has to be done to win over voters; Labour are all over the place in that regard.