First that visual debugging was still small and niche, probably not suited to the environment at Bell Labs at the time, given they were working with simpler hardware that might not provide an acceptable graphical environment (which can be seen as a lot of the UNIX system is oriented around the manipulation of lines of text).
This is different from the workplace where most game developers, including J. Carmack were, with access to powerful graphical workstations and development tools.
Secondly there’s also a difference on the kind of work achieved: the work on UNIX systems mostly was about writing tools than big systems, favoring composition of these utilities.
And indeed, I often find people working on batch tools not using visual debuggers since the integration of tools pretty much is a problem of data structure visualization (the flow being pretty linear), which is still cumbersome to do in graphical debuggers. The trend often is inverted when working on interactive systems where the main problem actually is understanding the control flow than visualizing data structures: I see a lot more of debuggers used.
Also to keep in mind that a lot of engineers today work on Linux boxes, which has yet to have acceptable graphical debuggers compared to what is offered in Visual Studio or XCode.
I also recommend anyone interested to have a look at the Erlang/OTP ecosystem, especially for their design decisions.
While the language and the platform isn't popular, the OTP team does present rich architectural patterns and ideas that can improve your design