Generally speaking to be a commercial proposition mass market lenses need to be made in large numbers, or for a niche market at a very high price. Therefore it is a matter of balancing production numbers against what those wanting the product will pay. The larger production run you can get the cheaper you can produce the product, but there has to be sufficient numbers wanting the product for that level of production.
As soon as you produce a niche item the price has to rise, then though there may be a demand for the product there may not be enough at that inflated price.
Nikon and Canon both produced exotic lenses in the 1970's-80's that have now been dropped from their inventory. In the old days such firms were largely run by optical engineers and it was often a case for some more exotic items of "look what we can do" or "vanity" production rather than being a sound commercial proposition.
In the 1990's with recession, reality bit in most industries and the former engineers or tradesmen then running the firms and deciding what the firm produced in the past were superseded by outside managers and accountants who decided if it does not make the maximum return possible from the production line and factory space it takes up we will drop it and produce something more profitable. So a lot of less profitable items were dropped by lens manufacturers.
As we know today, most firms are now often run by managers and accountants brought in from other industries whose aim is maximum revenue for the shareholders, rather than being steeped in the industries traditions through having come up from the shop floor, so I suppose they do bring a bit of reality to what it pays to produce.
Nikon dropped its 70mm-180mm macro zoom due to limited demand, plus its 5T and 6T close up lenses. Probably the Canon MPE-65 is a hang over from the old days pre-accountants in charge, but is kept in production because its development costs etc have now been recovered, or written off years ago. The question is would Canon produce it today if it started from scratch again? I doubt the market would be large enough, coupled with development costs, for the accountants now in charge to OK it.
In future it may not be why don't the lens manufacturers produce this or that macro equipment, but what else will they drop as uneconomic to produce for such a limited market?
http://www.photozone.de/nikon--nikkor-a ... est-report
http://www.bythom.com/70180Macrolens.htm
As has been said, there was little demand for the 70mm-180mm Micro Nikkor at its price when in production, so Nikon dropped it and now it goes for more secondhand than it did new in its declining years.
DaveW