I believe it's because Slovenians are more exposed to Serbo-Croatian than Serbians/Croats/Bosniaks are to Slovenian. I'm a living example, I've pretty much never been exposed to Slovenian language until I was an adult.In my experience, Slovenians can undestand Serbo-Croatian quite well, but this is not the case the other way around. What is the reason for this one-sided intelligibility, I don't know.
Were the three official languages really equal in ex-Yugoslavia ? Could a Slovene or a Macedonian live without knowing "serbo-croatian" (as the language was called at that time) ? Were official documents available in all three languages ?