Why The LUF, or lowest usable frequency, is the bottom end of the window for a given path: below it, absorption in the lower ionosphere (mainly the D layer, where collisions are frequent because the air is still relatively dense) soaks up the signal energy. The wave may still refract, but so little of it survives the trip that the signal is too weak to be useful at the far end. Absorption grows roughly as the inverse square of frequency, so the lower you go the worse it gets. The usable band for a path therefore lies between the LUF and the MUF.
Watch out Passing through the ionosphere is what happens above the MUF, at the high end of the window, not below the LUF, so that choice has the situation upside down.
MUF too high, signal escapes; LUF too low, signal gets eaten. Low frequency = high absorption.