The APAS docking system was first used on the Apollo Soyuz Test Project mission. Both parties (US and USSR) already had developed and used docking mechanisms by then. Is there any truth to the claim (see the comments in that question) that the US and USSR were unwilling to share details of their existing systems with each other, so they had to develop something new instead?
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Prior to ASTP, both Apollo and Soyuz spacecraft used a probe-and-drogue docking system, which requires different hardware at each end of the docking interface. This means that if you want two spacecraft to dock with the same space station, for example, the spacecraft can't dock with each other.
In order to support ship-to-ship docking, therefore, an all-new androgynous docking interface was required.
answered Mar 20, 2016 at 20:46
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$\begingroup$ But couldn't one of the ships just implement a dock instead? Especially that probe-and-drogue is still used on regular basis currently, so it wasn't like the standard changed. $\endgroup$– SF.Commented Mar 20, 2016 at 20:52
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$\begingroup$ What do you mean by "implement a dock"? $\endgroup$ Commented Mar 20, 2016 at 21:00
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$\begingroup$ Put the drogue / docking port on one of the crafts. $\endgroup$– SF.Commented Mar 20, 2016 at 21:34
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2$\begingroup$ Space station is female (i.e. equipped with drogue). Spacecraft A and spacecraft B are male (i.e. equipped with probe). You have an emergency situation where you need to dock A and B. This is the problem APAS was originally trying to solve. $\endgroup$ Commented Mar 20, 2016 at 21:38
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2$\begingroup$ Certainly the promise of APAS never materialized, despite it being used in one form or another on a few generations of ships. Passive-only APAS on stations combined with true androgynous APAS on ships would have been okay but the Russians never committed to it. Still, the question was "why was APAS developed/why weren't then-existing systems used", not "what happened to APAS after 1975". $\endgroup$ Commented Mar 21, 2016 at 16:49