The conditions of the miners and their families in Emile Zola depicts for us in Germinal are generally accepted as being realistic: life for the French coal miners was miserable, base, and short. These things, even more than the blatant unfairness of their treatment, is why they finally decide to strike.
The Maheus are the family Zola features prominently in this story, but they are representative of all the other mining families. The overwhelming picture of life for the miners of Montsou, represented by the Maheus, is one of deprivation. They never have enough food to eat, and at least one character, Alzire Maheu, dies of starvation.
[T]he cupboard was empty, the little ones asking for bread and butter, even the coffee was done, and the water caused colic, and the long days passed in deceiving hunger with boiled cabbage leaves.
They never get enough sleep, they share beds, and they even have to sleep in shifts. Their house is small and their routine is monotonous and hard. Money is scarce at the beginning of the story, and it will grow more scarce as time passes. The more family members who work, even the youngest children, the more money the family makes--and the quicker each person in the family begins his or her unfulfilling and painful life.
Because their lives are barely above the level of subsistence, the miners take their pleasures when they can find them. They are sexually active and not always particularly selective, they drink as much as their finances will allow, and they live coarsely in almost every way because of their living conditions. There is little for any of them in the way of fun or intellectual stimulation, and there is little for them to look forward to beyond just living to work in the dark for another day.
Because their lives are so physically demanding and nutritionally deprived, the miners do not live long lives--which may, in fact, be a kind of blessing. In the Maheu family, Vincent is an old man, but he has been fortunate to escape many accidents in his mining career; he is still alive but his life is unsatisfying and of course he goes crazy at the end of his life. His son, Maheu, dies during the strike. Zacharie is the oldest son and has already fathered two children; he will continue to work the mine until an accident happens or until his body gives out on him. Catherine dies in the mines, and eleven-year-old Jeanlin has already been crippled by mining.
He was so small, his limbs so thin, with enormous joints, enlarged by scrofula, that [Catherine] took him up in her arms. But he kicked about, his apish face, pale and wrinkled, with its green eyes and great ears, grew pale with the rage of weakness.
He is already depicted as an old man. Two others, Henri and Lénore, are too young to work yet, but their time, of course, is coming. The mother, La Maheude, worked early in her life and has to go back down into the mines at the end of the story just to support what family she has left.
The miners strike because they have no money but they know others are getting rich off their work and sacrifice. They strike because when they are not lethargic and passive, they are angry and aggressive--and with just cause. They strike because they think they might be stronger than those who run the mines (which they are, but of course the mining company brings in outside help). Mostly, though, it seems to me that they strike because they have nothing of value to lose. Literally nothing, including their lives.