Why we need waves below 30MHz for sky wave propagation

There are 3 types of propagation which are used and they are ground wave propagation, sky wave propagation and space wave propagation.

In skywave propagation the atmosphere layer is used to reflect radio signals to places far away from the transmitter on earth.

For frequencies which are lower than 30MHz, the atmosphere is opaque & will reflect the signal well. If incident angles are low we can transmit the signal far over the horizon. 

But for frequencies which are above 30MHz the atmosphere is not opaque at all. The waves now gets transmitted to outer space and don't get reflected now by the same atmospheric layer. So sky wave propagation cannot take place with such high frequency.