In Circuit Debug needs the Reset, ICSP data and ICSP clock pins available for the debugging, so generally they aren't available to be used by the application.
On a 14 pin chip, with power and reset, there are 11 pin remaining for the application. To lose 2 of those on every single item produced just to make the developer's life easier on a handful of prototypes could be seen as wasteful.
Microchip's solution was to make the pic16f688 with basically a spare ICSP data and ICSP clock pins, plus a few others, so that an application with a DIL14 socket could take the AC162056 directly while performing In Circuit Debug, with no hardware or software changes.
That was about 15 years ago. Many products use surface mount ICs, so there's no easy way of having a debug board that can be just plugged in. If a surface mount microcontroller is used, a debug board would have to be connected with wires, so a larger microcontroller could be used for debugging, and so there's no real need for a debug version.
Just having a 14 pin DIL IC could cost a lot more in board area, height and assembly costs than having a 20 pin QFN package, so a couple of spare pins taken to test points will cost nearly nothing and will allow a few wires to be soldered on to do everything that the debug board would do.
Depending on how the microcontroller is programmed, the ICSP data and ICSP clock pins may be needed on production devices anyhow. With DIL ICs, it was practical to program each one in a ZIF socket, but that's not done so often now and ICSP is used, so those extra pads are available for debugging anyhow.
In short, there's no need for separate debug versions any more, so Microchip don't sell them.
