A typical piece of paper is 8.5 inches wide and 11 inches tall. The page orientation of a typical piece of paper is portrait, or taller than it is wide.
A computer monitor is wider than it is tall. This means that a page full of text on screen has wider line lengths, and less vertical space. When a reader encounters longer line lengths they are likely to lose their place when moving from the end of one line of text to the beginning of another line of text.
Web copy is most useful when it has short line lengths and short paragraphs, so it is easy for a reader to keep their place. Focus on word choice and shorter sentences so it is easy for a reader to follow your ideas.