Why did Allah swear by the width of the earth and not its length?

There is no oath sworn by Allah concerning the width of the earth. What has been mentioned is a simile comparing the width of Paradise to the width of the heavens and the earth. The reason for this simile is that "width" denotes vastness and spaciousness. If width is mentioned, it implicitly indicates the existence of length, whereas if length is mentioned, it does not necessarily indicate width. The Arabs used this expression to signify vastness, as in their saying, "a wide land," meaning a spacious one. The verse intends to convey that Paradise is exceedingly vast, wider than anything you have ever seen.