Why isn't the hydroxyl group of citric acid deprotonated while the carboxyl groups are?
You are correct in saying that the carboxyl groups will have resonance structures after being deprotonated. Thus, the carboxyl groups are fairly acidic and will deprotonate in water. On the other hand, deprotonation of the hydroxyl group will not lead to any resonance stabilization, such that the hydroxyl group will be much less acidic than the carboxyl groups. In fact, the $\mathrm{p}K_\mathrm{a}$ of alcohols is usually in the range 16-20, which means that the alcoholic hydrogen will not deprotonate in water. It can deprotonate, however, in other solvents.