Maybe it's a matter of resources. If the same group of people is assembling the new ones, perhaps it's too expensive to employ more people just to make headphones they are thinking about phasing out. It's not like HFM will be incapable of manufacturing more HE-500s. I have no idea how long it takes to produce one pair, but if they've got current stock, it makes sense to focus on producing the new models. Once the initial demand is met and sufficient stock accumulated, perhaps they'll make more HE-500s, assuming depleted stock and continuing demand.
Again, without detailed knowledge of the production process and cycle(s), it's speculation. Yeah, I know the HFM rep said the plan is to discontinue the HE-500s. Plans change, eh? Seems to me a small company can be flexible in its decisions.
On the other hand, maybe the workers need to be re-trained each time there's a production run for a specific model. I read recently that some tube manufacturers needed to re-train their employees for six(?) weeks prior to a production run of tubes they'd made in the past. That was an eye-opener.