Just last year, Sennheiser released a successor to the long-running HD-25-1 II: the Amperior. The new additions were aluminum earcups (which improve the resonance), an in-line mic/remote, and new color options (blue and silver). All that sounds great, but the big problem was the price increase: $350 up from $200, which apparently didn’t fare well with consumers.
For whatever reason, Sennheiser has officially discontinued the product according to the official Amperior product page and a Sennheiser representative’s response to an inquiry.
I think Sennheiser was in a tough situation on this one, especially when it came to pricing. For most people, $150 for these slight improvements was simply not worth it. But if they lowered the price to ~$250, would it jeopardize the HD-25-1? That is probably the situation they wanted to avoid.
If you are still interesting in buying a pair, thankfully it will still be available on the market for another 6 months or so. Amazon currently has the best prices I could find: $218 for the blue version and $314 for the silver version.