Because it would make scrolling more frequent? For multicolumn text to reduce scrolling, the column height would have to match the available viewport height. And if your text exceeds what can fit in multiple columns on a single screenful, scrolling also becomes awkward, because you'd have to scroll exactly to the next screenful to have any consistency. Multiple text columns only make sense on extremely restricted layouts, or where the volume are entirely independent instead of a single flowing piece of text, or where there is still a direct horizontal relationship (like annotations or translations beside the main text).