Why is this mode of propagation limited to frequencies upto a few MHz?

In standard AM broadcast, what mode of propagation is used for transmitting a signal? Why is this mode of propagation limited to frequencies upto a few MHz?

In standard AM broadcast, ground wave propagation is used for transmitting a signal. Attenuation of surface wave increases very rapidly with increase in frequency i.e. why it is limited to frequencies upto a few MHz. In AM broadcast, range of frequencies, are limited to 30 MHz.