"The two-space convention is left over from the days of typewriters." And that convention itself came about as an imitation of common 19th-century typesetting

"The two-space convention is left over from the days of typewriters." And that convention itself came about as an imitation of common 19th-century typesetting style, where more space after punctuation was still fairly normal. Technological and aesthetic changes--the adoption of Monotype and Linotype machines, and the influence of early book designers like Bruce Rogers--started a trend to more even spacing in typeset printed books.