Even back then, the Soviet space program had the electronics to make this much less complicated. Remember, in 1986 they had the Buran shuttle which flew to the orbit and landed fully autonomously, under computer control.
The clock was probably developed much earlier, and then perhaps slightly modernized with LEDs and such. Developing a new clock for spacecraft would probably take a year or two, for all the testing and certifications, so nobody bothered.
Less complicated isn't always good. There was no problem driving the clock display with the onboard Argon-16 of course, but the clock was decoupled into a separate device to not have a single point of failure, and also to replace the old onboard chronometer which was even more complex and failure-prone.
This was my thinking as well. Spacecraft are already incredibly complex. Building a new clock for the sake of building a new clock surely wasn't a priority.
The clock was probably developed much earlier, and then perhaps slightly modernized with LEDs and such. Developing a new clock for spacecraft would probably take a year or two, for all the testing and certifications, so nobody bothered.