In the first place, both the SNP and Labour consider themselves "Scotland's Party". They are opposed, certainly, to the Conservatives but they're also opposed to Westminster, regardless of which party is in power in London. Their task is to "defend" Scotland come what may even if that means insisting that sensible public-sector reforms in England are not necessary in Scotland.It's a good article which also touches on the astonishing (to this outsider anyway) level of enmity between the SNP and Scottish Labour, which I think is unmatched (even in good ole N.Ireland) in UK politics.
But of course it doesn't end there. Labour see the SNP as upstart usurpers, threatening their cherished position as the "natural" party of government. The SNP membership, meanwhile, correctly view Labour as a Unionist party. With support for independence no higher now than it was when Salmond first strode into Bute House it may seem as though the constitutional question has sidled into irrelevance but it's certainly not irrelevant to the true believers on either side