Why A PIN diode has a thick undoped (intrinsic) layer between the P and N regions, which widens the junction and keeps its capacitance very low. Low capacitance means that when the diode is reverse biased or unbiased it looks like a near open circuit even at RF, giving good isolation; forward biased, the stored charge in the intrinsic layer makes it act like a low, current-controlled resistance. That combination of high off-isolation and low on-resistance is exactly what an RF switch or attenuator needs.
Watch out High reverse breakdown voltage and power dissipation are useful ratings in rectifiers and power devices, but they have nothing to do with how cleanly a diode passes or blocks RF; the claim about reverse bias controlling forward voltage drop is not a real diode behavior.
PIN = thick Intrinsic layer = low capacitance = the RF can't sneak across when it's off.