This is primarily because most of the content that we consume nowadays is shot in a horizontal orientation. Most people tend to take videos in the landscape orientation or horizontally. For comparison, a landscape video would have a resolution of 900 x 600, meaning it would be wider than a vertical video. A vertical video will have a resolution of 600 x 900 or higher with a 9:16 aspect ratio. They are ideal for viewing on a smartphone as they have such tall displays. These videos are taller instead of wider, and they usually have an aspect ratio of 9:16.
A vertical video is also known as a portrait video.
