Why The MUF is the highest frequency the ionosphere will still refract back to earth on a given path, and the LUF is the lowest frequency that can get through D layer absorption with usable signal strength. Usable skywave frequencies lie in the window between them, LUF at the bottom and MUF at the top. If absorption rises (or ionization falls) until the LUF climbs above the MUF, that window closes and there is no frequency that both survives absorption and gets refracted back, so ordinary skywave contacts over that path fail.
Watch out The choices about enhanced propagation invert the idea: a wide spread between a low LUF and a high MUF is what makes conditions good, and nothing is enhanced when the two cross.
Usable band = LUF to MUF. When the floor rises above the ceiling, the window slams shut.