It'd depend a lot on the licensing situation of each brand.
Take the Rams for example; Steve Mascord owns their IP now.
So the NRL and Sekem and/or Tidwell (the companies that currently produce the NRL's retro merch) would need to make a deal with him to produce any Rams merch, which, unless he's a very charitable or stupid man, would make them significantly more expensive to produce than other merch ranges. That extra cost would have to come out of somewhere, for example be passed onto the consumer, eat into profit, etc, you get the idea.
That extra cost alone will likely drastically reduce the odds of them ever looking into producing Rams merchandise, to the point that there's probably a better chance of Steve producing Rams retro jerseys than the NRL. Though if Steve produced them they'd almost certainly be without NRL/Super League badges and sponsorless, which is probably why he hasn't done it already, but I digress.
With the exception of the Seagulls I don't know about the current IP ownership of any of the other defunct clubs from outside of NSW. However I imagine that it makes at least a few of them much more difficult for the NRL to produce merch for, and I'd be willing to bet that it's a major reason why they haven't produced any merch for some of the potentially more lucrative ones yet.
