This is just a guess but I think AMA doesn't stock injectable winny because most don't inject it because of how painful it can be. Most use injectable winny as an oral. So instead of making oral winny and injectable that most will take orally anyways, it's more cost effective and time effective to just make the one that more customers will use. There could be another reason, this just made sense to me in a business way of looking at it.
When items become oos, the eta is usually somewhere around 2 weeks. Sometimes sooner sometime a little longer. When I find out I'll let you know.