20kW is a lot for HF because it's a "planet scale" frequency band. VHF signals fly off into space when they hit the curvature of the earth, but HF signals bounce off (technically are refracted by) the ionosphere, sending them back down to earth. Thus, VHF is only "line of sight"; if someone off the horizon abuses VHF, it's not a problem for your VHF communications on the same frequency.

HF signals can be reflected by the surface of the Earth after hitting the ionosphere and repeat this process until they've gone around the world (sometimes more than once!). This is not a rare quirk; amateur radio operators take advantage of this on a daily basis, with an absolute maximum power limit of 1500W. (But 1500W is uncommon; my radio does, at max, 10W. And I can communicate with Europe from NYC fairly regularly.) The result of all of this is you have to be careful about HF signals; you can ruin it for everyone in the world if you make a mistake. VHF, whatever.

VHF does occasionally propagate over long distances. It used to happen with analog TV and was quite the rare and amazing sight to behold (I've never seen it).

https://en.wikipedia.org/wiki/Tropospheric_propagation#Tropo...

https://en.wikipedia.org/wiki/Sporadic_E_propagation

TV moved from VHF to UHF during the digital transition, so this doesn't happen anymore.

Finally, there is a lot more VHF frequency available than HF. For example, one band that amateurs have is the 40 meter band, which is 7MHz to 7.3MHz. That is 300kHz of bandwidth for everyone in the world. Meanwhile, just 1 FM radio station is 270kHz!