Also, I found it out too and I hope they don't leave out covers in future manga, especially not Days or KH2.
Days will be individual volumes unlike these omnibuses, so they won't be leaving any covers out. And I assume KH2 will use color pages for the covers not used for the first two omnibuses and Volume 3 (which will most likely be the recently released volume 6 in Japan) and on will retain their covers.