One reason is that the FFXII OST is commercially a flop. It sold less than other soundtracks in the main series (No, don't even talk about FFX-2; I recently knew that one has actually the HIGHEST sales number in the series because of Komi Kuma the popular J-Pop singer.

)
Another reason is that FFXII's tracks are a lot harder to arrange into piano form, as far as I know.
No, the tradition certainly have not ended. They have their FFVII and (even) FFXI Piano Collections. Maybe some years later we can actually see one. ^_^
Given the fact that FFXIII's music production is given to Masashi Hamauzu the vgm piano master

we can hopefully see a Piano Collections album.
Jam it back in, in the dark.