There are several issues at large
We only have about one a week and yes good images don't get showcased, we would like a block of 4-5 recent showcases, not ideal but better than the one currently, this is being looked into but requires a code change which Hugh, Ruth or I cannot do.
We try to vary it so that the same people don't get showcased repeatedly, dA have a similar policy with the DD's
If a member removes a showcased image from their gallery, that also removes the showcase, so you may have a showcase that is no longer a showcase.
We have said in the past that we will consider suggested showcases, but this forum is to highlight those that anyone feels is showcase worthy.
Showcases are picked by one of three moderators, it is subjective, we rarely have our work showcased as we feel it is mostly for the members and don't generally feel that we should showcase one another's work, regardless of whether it is worthy or not.
Personally I have no problem with doing more showcases, so you will have to go look in the showcase gallery to see them, the one a week format was so that each one had a fair amount of time on the home page, if the majority want more, fine and no problem as far as I am concerned.
The showcase gallery was removed because of people pickling holes in the process, it was returned because I requested it and offered to take it on myself because I think its a good feature, Hugh and Ruth suggested that we all do it, Sometimes I can see why the people that were previously doing it, got fed up with it.
I have suggested several DD's that have not been showcased but lesser images have, so I'm off to dA to have a moan about it.