Why Reactance is the opposition that capacitors and inductors present to alternating current, and like resistance it is the ratio of voltage to current, so it is measured in ohms. Formulas bear this out: inductive reactance XL = 2(pi)fL and capacitive reactance XC = 1/(2(pi)fC) both work out to ohms. The difference from resistance is that reactance stores and returns energy rather than dissipating it as heat, but the unit is the same.
Watch out The farad is the unit of capacitance itself, not of the opposition that capacitor shows at a given frequency; the siemens is the unit of conductance or admittance, the reciprocal of ohms.
Resistance, reactance, impedance: all three are volts per amp, so all three are in ohms.