To quote Robert Bringhurst's The Elements of Typographic Stlye: "Anything from 45 to 75 characters is widely regarded as satisfactory length of line for a single-column page set in a serifed face in a text size. The 66-character line (counting both letters and spaces) is widely regarded as ideal."

Essentially, the theory is that longer lines are harder to read. A US Letter or A4-width page is just too wide for normal body text size according to most professional typographers. LaTeX provides typographically sound defaults.