Let me elaborate,
Green Party want environmental policies promroted but in becoming a "mainstream" party and embracing another political agenda the passion for the environment becomes diluted.
Scottish National Party want independence but have had a minority government in Scotland during which they have achieved much against strong opposition. An SNP government will have a referendum during the next term of office, if there is a majority for Independence, what then? Scottish political parties would no longer have a mandate for representation in London and would return to Scotland to campaign for seats in Holyrood. Should a party set up for one purpose, not disband and allow their members to find their natural place?