Maimonides (Hiring 2:1):
The three laws stated in the Torah regarding the four bailees apply only to movable property belonging to an ordinary Jew, as it says, “money or goods,” which excludes land; and slaves are excluded because they are compared to land; and documents are excluded because their substance is not monetary value; and consecrated property is excluded, as it says, “when a man gives to his fellow”; and the property of a gentile is excluded. From here the sages said: with regard to slaves, documents, land, and consecrated property, an unpaid bailee does not take an oath, and a paid bailee or a renter does not pay. But if he formally accepted responsibility, he is liable for them.

And Nachmanides also writes this together with the laws of bailees.
And just as they derive it from the fact that it says “his fellow,” which is generally said about commandments that are specific to Jews.

If they are right, what is unique about the laws of bailees?

You meant a Jewish bailee guarding the property of a gentile. That is a different issue. I don’t know why there is a difference. Different suggestions can be offered. For example, the laws of oaths may not be relevant to a gentile, who does not believe in the Holy One, blessed be He. Alternatively, when a gentile deposits something with a Jew, that is a legal dispute between a Jew and a gentile, and in Jewish law every such dispute is judged according to the Noahide laws. A loan to a gentile is also treated differently. So is stealing from a gentile. Every legal relationship between a Jew and a gentile is different.