In the Mishna 38a, Rabbi Eliezer says you can make an eiruv for Y"T and Shabbos, one for mizrach and one for ma'arav.
The Gemara's discussion of this says in the name of the Rabanan that just as you cannot make an eiruv for half a day to tzafon and half a day for darom, similarly the case of Rabbi Eliezer won't work.
I wondered why the Gemara changed direction from mizrach/ma'arav to tzafon/darom. Then I noticed in our recent Gemara, 49b, the Gemara uses tzafon/darom as the model for chamar/gamal. Further along, on 60a, the Mishna uses mizrach/ma'arav for a permissible eiruv.
Is it possible that there's a pattern here of using one set of directions for permissible eiruvin and another for problem eiruvin? If so, why and if not why the change on 38a?
You bring up an interesting point. However, see Eruvin 82b where north-south is used and there is no "problem". (The Eruv is with his mother to the south.)
Also, on Eruvin 60 (east-west) the Eruv doesn't work if it's too far away.
Another point: Rabeinu Chananel on Eruvin 38, the Rabanan's opinion reads "north-south" both times.
Actually there is no Mishna in Eruvin which uses north-south. But see Peah 6:4 and Sanhedrin 59b. where north-south is used. In Crisus 7b east-west is used.
In conclusion, we're not sure there has to be a reason. Each Tanna or Amora can use his own language.
All the best,