The reason for this is fairly simple to explain. Mincha is the afternoon prayer. Maariv is the evening prayer. Although there is much dispute regarding the exact definition of ‘afternoon’ and ‘evening’, one thing is clear: they can’t be recited at the same time, because it’s a self-contradiction (‘tartei de-satri’). Thus, I might believe that the very late afternoon, after plag ha-mincha (1.25 adjusted hours before sunset), is already evening, but then I can’t daven mincha then. Thus, if one wishes to daven mincha and maariv consecutively, one must be careful that one of the potential dividing lines between afternoon and evening – plag ha-mincha or sunset – falls between the two services.
The question that I have is as follows. This line of reasoning makes perfect sense for weekday mincha/maariv minyanim. For Shabbat, however, another factor should come into play. On Shabbat, we have a concept of mosifin min ha-chol al ha-kodesh. We can voluntarily make Shabbat early, extending its sanctity into the Friday afternoon daylight hours. It seems to me that this acceptance should generate a significant enough ‘dividing line’ between the afternoon and evening. Once I accept Shabbat, it is a new day – it’s Shabbat, not Friday. The day itself generates an entirely new set of commandments and restrictions, including new obligations to pray special prayers. One can argue that it may be Shabbat, but it’s still not ‘evening’. I find that argument unconvincing; Accepting Shabbat means that the evening has commenced early (and if one forgot to daven mincha – or, better yet, davened the wrong mincha after plag, and must repeat it after having accepted Shabbat – what should he say? Weekday mincha? It’s not a weekday anymore! Shabbat maariv twice? It’s a tarti de-satri! Wait until after sunset and then say Maariv twice? It seems absurd that one would be in a situation that he CAN’T daven – and R’ Yochanan would certainly find that possibility repulsive! Shabbat Mincha? While that possibility may exist theoretically, it seems completely absurd. What’s the answer? Shabbat Maariv, and it’s not a tarti de-satri for the reasons outlined above). One can recite the evening services without concern that he is engaging in self-contradictory behavior, because the first service was before Shabbat, and the latter one is on Shabbat.