What parasitic characteristic causes electrolytic capacitors to be unsuitable for use at RF?
Why Real capacitors are not pure capacitance. An electrolytic is built from long strips of foil and paper rolled into a cylinder, and those turns plus the long leads act as a series inductance. At RF that parasitic inductance (along with high equivalent series resistance) dominates, and above the part's self-resonant frequency the capacitor actually behaves inductively, so it no longer bypasses or couples RF.
Watch out Dielectric leakage is a real electrolytic flaw, but it is a DC/low-frequency loss (self-discharge) and is not what kills their RF performance; skin effect describes current crowding in conductors, not a capacitor parasitic.
Rolled foil = a coil in disguise: electrolytics fail at RF because of series inductance and self-resonance.