The reason not to use two spaces has to do with creating an even typographic colour to the page. Colour here does not mean hue, but that the page has an even, consistent appearance, with nothing catching the eye and disturbing the reading process.

Bold type within text destroys colour. It makes the eye jump ahead to it. While this is fine for key points in a textbook, in a work meant to be read linearly, it steals the eye, causing the reader to visually stumble. It is not as bad in a heading, where the eye is trained not to be surprised by the blackness (although some book designers will not use bold for heads either, to maintain colour).

A double space does the same thing, in reverse. Instead of blackness stealing the eye from the textflow, it is the whiteness of a bullet hole of white on the page. It causes the eye to jump forward to the end of the sentence to see what's caused this, and then having to find the place it jumped from, delaying your reading by a fraction of a second.

Other ways to destroy color include using all capitals (use true small caps instead) or using lining numerals (use old style figures instead). In each case these make a blob on the page that results in slower reading.