Why does water not enter the bottle in the inverted position?

Hint: Recall the factor which is opposing the water to enter the bottle. Explain the effect of pressure from underneath the bottle opening. Also, one can observe the process when we pour the water into the bottle in the usual position.

Complete answer:
We know that empty bottles contain air inside it. When we invert the bottle and deep it into a vessel containing water, the water will try to enter the bottle from underneath. Thus, we can see the inside air has nowhere to go because the opening of the bottle is covered by the water. As water rises into the bottle, it compresses the air inside it. The compressed air exerts equal pressure on the water and will not let the water go deeper into the bottle.

Thus, we can say the air pressure inside the bottle restricts the water to get inside the bottle. If we assume the bottle is in its usual position, that opening is from the upper end. When we pour the water inside it, the air inside the bottle comes out eventually and makes a room for the water. If the bottle is in the inverted position, the air has nowhere to go and it will oppose the water. It is not always true that the water does not get inside the bottle in an inverted position. If the water pressure is greater from underneath, it will compress the inside air as we push the bottle down.

Note:
 If the bottle has a small opening from the above, the air will come out from this opening and water can rise in the bottle. If we push the bottle in the swimming pool or a large vessel containing water, due to huge pressure from underneath the swimming pool, the water will rise into the bottle to greater extent.