The Gemara, Sanhedrin 76b, states that "one who returns a lost object to a כותי (or, per the marginal notes there, גוי) is censured by the Torah. Rashi there explains: "He is thereby equating the כותי with the Jew, and demonstrates that he does not consider returning a lost object to be the subject of his Creator's commandment, since he is returning it even to a כותי, about whom he wasn't commanded."
Now, whether this rule applies nowadays, and to whom, is a whole other issue which I'm not getting into right now. My question is simply this: We know that there are other Torah laws in which similar distinctions are drawn - for example, the mitzvah to lend to a Jew without interest (as contrasted with a non-Jew), or the rules of לא מורידין ולא מעלין (Avodah Zarah 26a-b and from there in Choshen Mishpat 425:5). So why does the Gemara single out "returning a lost object" more than any of these?