I remember MLA format always being our English Bible.
Back when I was in school, MLA style was basically
2 spaces after periods and colons (where it's just a dot), 1 space after commas and semi-colons (where it's a dot and a tail). This was to differentiate them on typewritten font and make them easier to distinguish.
With computers and the internet being so prevalent, written works aren't seen nearly as much, and different styled fonts are all over the place. For internet usage, one space is preferred since space on a computer screen is a limited commodity. (With papers, you can always add pages, so a few extra spaces on a single page doesn't make a difference. But online, there's only a limited amount of space on the screen before you have to scroll, and quite honestly a lot of people don't read what they don't NEED to read online, so if they need to scroll, a lot of people will go to another website if they don't feel that more information is really necessary. Similar to newspapers, you'll read the partial story on the front page, but a lot of people won't finish the story somewhere on page E-13. ...But newspapers use multi-columned justified spacing, so they don't necessarily follow the 1-2 space rule, either.)
There's a bunch of studies and theories on how people read and what will make them read more. I'm not sure if the reasons I stated above were necessarily the reasons why the rules changed, but they do support the change.
Adrienne, funny you should say that, when I took my typewriter keyboarding class (ugh I'm old) 3 spaces was occasionally necessary. When you're trying to make something stand out, you'd add spaces:
Where there's 1 space between each letter, and 3 spaces between each word. (Typewriters didn't have a "bold" or "italicize" option.) The newer word processing programs have options to expand/condense the amount of space between each letter so you don't have to literally add extra spaces.