The reasons are severalfold.
First, OLD technology. Cavity tubes are going the way of the dinosaur. The new solid state units are far superior...no warm up time, cheap to repair (cavity tube txpdrs can be really expensive to repair).
Second, Narco went from being the leader in avionics some years ago to being a virtual non-player. Dealer networks vanished, support vanished, and product development....well....just look at their stuff!
Until they come out with some new stuff that has new technololgy, uses serial communication and actually are willing to support customers then I think you'll see them slowly dissapear. Machined D-Sub connectors are also so much more enjoyable to work with then the old molex card connectors.
It's rather ironic that their biggest market it is "plug & play" retrofits with King KT's (which there are also a pile of on the used market) and old Cessna ARC's. It's ironic because King also seems to be trying really hard to get out of the GA market. Try to get customer support from them...I kind of lump both companies together when it comes to good support or good products.....it's kind of sad..from the 50's to the 70's 90% of all avionics were Narcos. Then from the 70's to the 90's everything was King products. Now....well it's Garmin for the time being, until a competitor emerges. (it's not IF, just when).
Anyway, those are my observations and 2 cents.
Cheers,
Stein.