I imagine it's simply complex. Part of the box model involves expanding containers to fit the content within them, and properties like float, overflow, width, and height run counter to that. So what do you do with "expand this div to fit the text it contains, but also allow the text to be truncated if it doesn't fit, and remember to leave room for the ellipses"?
When you get into fancier concepts like truncating a set of lines, or only at certain word/phrase/sentence boundaries, you're into desktop publishing typesetting. To do that properly you're not just truncating when the container overflows, you're also adjusting the spacing between words and letters to make every line look consistent and to get as much of the text to fit neatly as you can. There aren't many pieces of software that professionals trust to do that well, and the ones that exist are expensive. We're not likely to see those algorithms getting into everyone's browser.