Why is there a gap between 7 MHz and 13 MHz?

[OgreVorbis]

When looking at the pirate radio logs here as well as the list... https://www.hfunderground.com/wiki/List_of_Pirate_Radio_Frequencies

You can see there is nothing noted in this range. I can understand why much higher frequencies are not used cause they are not very reliable. But in this range, I would expect easier antenna installation with a higher efficiency at lower heights. Also much shorter for smaller lots and increased daytime range. Maybe the doughnut is too big (bad local)? By doughnut, I mean the empty area between the groundwave and the skip zone. Is there just no space for it? I don't know this band too well.

[Brian]

A couple of Euros pirates do use frequencies above 7500. FRSH and JVG to name 2 use 77xx.
When the band is working. reception can be excellent at about 300 miles and more. A couple use 12 Mhz but I've never had much success rxing them.

[KaySeeks]

When looking at the pirate radio logs here as well as the list... https://www.hfunderground.com/wiki/List_of_Pirate_Radio_Frequencies

You can see there is nothing noted in this range. I can understand why much higher frequencies are not used cause they are not very reliable. But in this range, I would expect easier antenna installation with a higher efficiency at lower heights. Also much shorter for smaller lots and increased daytime range. Maybe the doughnut is too big (bad local)? By doughnut, I mean the empty area between the groundwave and the skip zone. Is there just no space for it? I don't know this band too well.

Propagation conditions are generally just not amenable for low-power transmitters at those frequencies now nor in the last few years. As the sunspot number generally increase over the coming 3-5 years, there will be more and more periods where the conditions will be better for such things.

Do some reading on shortwave propagation to understand this.