Its partly about energy dissipation, information theory and entropy. Also circuit design and practicality.  There is a thing called the von Neumann Landauer limit that for irreversible operation for one bit you have to dissipate $kT\ln2$ of energy. Current transistor expend much much more energy per bit. In the future though perhaps one can approach that limit. So people do think about and design reversible gate using classical devices. Some argue that these types of logic may help some computer architectures, even when the individual devices are not near the the theoretical limit.

People interested in these types of topics talk about adiabatic computing and charge reversible computing and the amount of entropy increase circuit operations have. 

The quantum gates I think are reversible because it is a coherent system, but once the wave function collapses you probably have an increase in entropy if you have performed the equivalent of a logic operation.