When was the last time characters' shards has been rotated out of shipments? Perhaps my memory fails me, but as far as I can recall there has been only additions for a long time.
If it is so, each time a new character is made available in shipments it occupies the same spots, progressively shrinking the chances to actually be able to buy them without refreshing the store.
Honestly, I'd say that buying from shipments is already crystal intensive enough without adding roulette style refreshes on top of it. Besides, if this isn't addressed, sooner or later it will inevitably reach a point in which the newest characters chances to appear will be small enough to be close to irrelevant.
Am I missing something? Perhaps chances to appear aren't equal among characters and are instead highered or lowered depending on how long they've been made available? It doesn't feel like it to me, but hey, no one is completely immune to bias, right?
Res non verba