Quite ironic that you’ve made a complaint about language subtielties, straight after using the phrase “two party state” in reference to Scottish polling (and the way that term is loaded is nicely highlighted by the way that hoosgs annexed amended it to “one-party state”.) ‘Two party system’, would be the more usual term.
Also, you’ve gone from “the SNP still don’t seem to be a particularly socialist party” to “no actual leftist party”. In the same sentence! So (no doubt unintentionally) your point is somewhat muddled. The SNP don’t pretend to be a socialist party, and are unlikely to every morph into one. But that doesn’t mean they’re not ‘leftist’. We can argue all day long about the specifics, but the SNP self-identifies as ‘centre-left’, and is broadly - broadly - accepted as such (excepting dafties that cling to the tartan Tories line, or who want to warp 1979 into something it wasn’t).
Also, be reminded of how appalling and incompetent Scottish Labour are perceived to been for the last decade or more (in comparison to the continued establishment of the SNP as, if nothing else, a safe pair of hands). The damage was done and the SNPs position established before Corbyn’s socialist credentials could rescue them. Boiled down, Corbyn’s positives don’t easily fit into the Scottish political landscape, and that isn’t helped by the self-destructove tendencies of Scottish Labour.
Also, be aware that Ruth Davidson was very effective at hoovering up the Unionist vote from that ‘Blairite’ Scottish Labour rump. Scottish Labour haven’t handled the constitutional questions well.
I’ve generalised and whatnot, but to anyone with even a basic understanding of Scottish politics, Labour being the third party is entirely explainable.
(No doubt some tryhard will use this opportunity to suggest that I’m making the case for Scottish exceptionalism. Don’t go there, kiddo.)