Cause it's so much better to get along with each other than attract new folks and win.
While getting along is one good value, I would note that the most extensive union organizing in American history came when the AFL and CIO were seeking to cut each others throats organizationally, fighting for position, and being driven to new organizing to attract new people to achieve dominance over their rival.
The issue is not unity with other leftists per se but unity of the working class-- while one may at times serve the other, they are not inevitably linked. Folks who denounce working with Democrats really don't have much credibility in suddenly discussing the wonders of unity-- it's all so selective rhetoric that it's hard to take too seriously.
-- Nathan Newman