Chapter: Drinking Nabidh and covering vessels

I came to Allah's Messenger (ﷺ) with a cup of milk from Naqi' which had no cover over it, whereupon he said: Why did you not cover it? - even if you had covered it only with a stick. Abu Humaid said that he had been ordered that waterskins be tied during the night, and the doors be closed during the night.

Abu Humaid Sa'idi reported through another chain of transmitters that he brought to Allah's Messenger (ﷺ) a cup containing milk, but there is no mention of the word" in the night

We were with Allah's Messenger (ﷺ) and he asked for water. A person said: Allah's Messenger, may we not give you Nabidh to drink? He (the Holy Prophet) said: Yes (you may). He (the narrator) said: Then that person went out speedily and brought a cup containing Nabidh, whereupon Allah's Messenger (ﷺ) said: Why did you not cover it? - even if it is with a wood. He said that then he drank it.

Why did you not cover it even with a wood across it?