There are a variety of reasons.
1. If you are outside Scotland it is perhaps easy to forget that the SNP are not just a fringe party - they are the government in Scotland for all the devolved powers, which are increasing following the referendum result, and I'd have to say they have done a reasonable job of it.
2. The SNP here in Scotland (and I haven't ever voted for them as yet) are clearly more competent than Scottish Labour of late, who are the only serious opposition to the SNP here, but they have imploded in recent years due to poor leadership and links with the main UK Labour party. I doubt that parachuting in Jim Murphy will be enough to save them.
3. The result of the referendum last year, and the frantic last minute interventions by Gordon Brown etc, might have achieved a No result but not by a decisive margin given that 45% of Scots wanted out of the UK. Independence is still an aim for many Scots, as the post-referendum rise in support for the SNP since then suggests, and it seems that those Labour supporters who voted Yes last September (as I did) will now switch their vote to the SNP in May (as I will).
4. The situation as regards Scottish MP's in Westminster is that should the SNP gain substantially from Labour here in Scotland the shift in numbers could be sufficient to deny Labour either an outright majority or enough seats to go into a coalition: the Tories are not a factor here, and the Lib-Dems are likely to be routed as punishment for getting into bed with the Tories, and depending on the number of SNP seats won then the SNP could have a greater influence in Westminster.
5. We are being told here that a vote for the SNP in May will keep the Tories in UK power but, and speaking for myself I think it is a risk that I'll take since I couldn't bring myself to support Labour any more. A friend of mine has a theory (his not mine!) that should the SNP weaken Labour here in Scotland, and should Labour do badly in England, that a Tory government might well encourage Scottish independence as a means of retaining power in England, Wales and NI - in that they know they have no prospect of being a major force here in Scotland so that removing Scottish seats from Westminster might give the Tories a long-term in-built majority elsewhere in the current UK.
Whatever happens it should be interesting!