Perhaps because it is a form of tribalism that requires absolute loyalty to the collective - also observed in sports and, yes, labor organizing. Some sociologists link it to the pre-modern mode of production, in which the household was th eprimary unit of production and consumption. Consequently, any form of individualism was disruptive to the integrity of this unit and thus discouraged (by both social sanction and economic deprivation, as those outside the household had very limited access to the economic means of subsistence). This tribal-like loylatly survived to the modern times and formed the basis of worker solidarity in the earlier stages of industrialization (woring class was drawn from peasantry).
However, as industrialization and progressed, that tribal solidarity eroded as a result of serveral factors, such as economic rewards for smaller sizes of household (children wre no longer an asset but a burden), increased production made individual (as opposed to collective) gratification easier, or geographical mobility which weakened social bonds. The farther a person moved from his/her peasant/workingclass roots, the weaker the sense of tribal solidarity. This is why the popularity of both group sports (football - both American and European variety) and collective violence (warmonering, support our troops and our home-team) is particularly popular among the lower class males.
Another factor is male bonding which emphasizes hierarchy and submission/domination relations - you have to submit to the alpha- male and dominate as many other males as possible, you have to be loyal to the male collective, whether it is a gang, a home team, or "our" troops. Women tend to form more personal and reciprocal relationships, whereas males tend to be psychologically defective and cannot function well without the collective - so maminatning the collective and group solidarity that holds it together is much more pronounced in males, and lower class males in particular.
Wojtek