The SNP will continue to do well in elections for as long as Labour deny that there is a significant proportion of their voter base who want independence. I would consider myself to be a natural Labour voter, and voted for them in my first general election in 1997. I was never a massive fan of Blair, but I supported many of the policies of the first New Labour government - minimum wage, 10p tax rate and devolution. Labour lost my support after the military actions in Afghanistan and Iraq and, while that was quite some time ago, they haven't done enough to persuade me to come back.
In terms of actual policies, the SNP have been more or less aligned with my views than any other Scottish parties since, and I have consistently voted for them in constituency elections for over a decade now. I wasn't really all that bothered about independence at first, but my view on that subject has gradually changed, probably more so since the Tories took power in 2010. Any chance of me voting LibDem again also evaporated at that point in time.
My vote in general elections goes to the SNP because the Tories are second place. In Scottish Parliament elections, it has been SNP/Green for the constituency and list ballots respectively. So at this moment in time, I have the government that I actually voted for, something that hasn't happened at Westminster since 1997.
Having said all that, nobody owns my vote. The SNP have done some things that I haven't approved of - centralising police and fire and rescue services, for example. On the other hand they've done some things that I have approved of, such as scrapping prescription charges and tuition fees. I don't see them as flawless, but many others seem to think they should be. Show me one political party that doesn't have its issues.
In the meantime, it's the party that a) best fits many of my views, and b) actually has sufficient weight in parliament in order to govern. Alba is still relatively unknown and fairly niche. The Greens simply don't have the numbers to be able to govern.
Leaving the Independence issue aside for a moment, the SNP is really no different in outlook to how Labour was in the early 1990s.