Seforno: Because poor people commonly look after them for pay.

Bava Metzi'a, 57b: To learn a K'lal u'Perat u'Kelal. Refer to 22:8:2:1.

Yerushalmi Shevu'os, 6:6: We learn via a K'lal u'Perat u"Kelal that whatever is subject to a. breakage, b. being taken captive, c. death, d. K'nas ¹ and e. Ona'ah (overcharging) is included in the Din of Shomer Sachar; to preclude Karka'os - that are not subject to a., b., & c.; Avadim, that are not subject to d., ² and Sh'taros that are not subject to e., for which a Shomer Sachar is not Chayav to pay if they are stolen or lost.

Ramban: 'It dies naturally, falls off a cliff ¹ or is captured by armed bandits'.

Rashbam and Targum Yonasan: 'It dies naturally, is killed by a wild beast or is captured'.

Mechilta: The Torah inserts the case of Meis (bi'Yedei Shamayim) to teach us that, Nishbar and Nishbah - like Meis, which the Shomer is helpless to prevent, he is only Patur if he could not possibly have prevented it, but Chayav if he could. ²

Rashi: Because a Shomer Sachar is Chayav if the article is stolen or lost..

Rashbam and Targum Yonasan: It means that there are no witnesses to testify (but if there are witnesses, he is obligated to bring them, and is not Patur with a Shevu'ah - Bava Metzi'a, 83a). ¹