The SNP are, essentially, a blanket party; they cover a large electorate and a lot of ground politically. Although, historically, they are a left-wing nationalist party, they in fact pick up a lot of voters from both sides of the Venn Diagram, so to speak; right-wing nationalists and left-wing unionists. A vote for the SNP is no longer much of a vote made to seek a particular target (at least until they get another referendum, which could be relatively soon), but a vote for the incumbents for want of any kind of alternative; they were recently up against the Tories and thinly-veiled Blairism in the form of Kezia Dugdale.
I'm not saying that people don't vote SNP for genuine reasons - of course many people do - but it still strikes me that the party's basic structure and aims have changed quite a bit recently. Perhaps, if there were more competent opposition that tackled one half of that Venn Diagram (the left-wing bit, probably) then, if they were able to hold Holyrood to account well enough, they could quite possibly start to hoover up wavering voters as well. Heck, sometimes the SNP don't even look that left-wing (though, I admit, sometimes they do) - is it possibly that, now that they hold the balance of power and are beginning to affect politics over the border, they have a slightly more centrist agenda?