Roger, are voters in Scotland or indeed the UK as a whole spoilt or spoiled for choice?
Perhaps the voters of Scotland simply did not wish to vote for a party allied to, connected to or controlled by a main group at Westminster?
Maybe the SNP simply fielded enough decent candidates and the other Partys less so?
The SNP do not have it all to themselves in any case...
http://www.scottish.parliament.uk/msps.aspx