Pleaselog inorregisterto add a comment.
The reason is simple: that’s the only (modern) official art that exists. Theoriginal art from Gold/Silveris actually a G, but after then the official art has used the F form. There is not official art for the other letters.
Even thePokedex on the official siteuses the F form art.
The real reason:According to my memory, when Unown was revealed, it was revealed in it’s F form, and shortly afterward, the other forms were revealed. So Unown-F is the original Unown and therefore the original artwork should be on the PokeDex page
Source: Memory
I really don’t think this is a fact to question. My only true answer is because all letters A-Z, !, and ? all have the same stats and move pool and if Pokemaster were to add extra tabs for all 28 symbols similar to Mega-Pokemon then he/she would have to find 27 more pictures. In all there’s no point to add all 28, all Unown are exactly the same besides the fact that they all look different. For why it’s and F? All I can say is maybe Pokemaster likes the letter F.