why we not say that x-1 instead of 1-x .

If this is related to equilibrium chapter and reactant side, then I must tell you that we have to say it only 1-x because initially, we supposed that reactant has only 1 mole of it.And after some time, means at equilibrium,if x moles are used up of reactant to make product,then moles of reactant would be written as 1-x because, obviously, x is smaller than 1 in this case.And if you write x-1, you will get negative number of moles, which is not possible.