Tradition and/or to forestall grognard rage. Seriously, I'd suspect that a very substantial portion of the playerbase would houserule stat-rolling back in anyway; they might as well have an official way to do it in order to keep things mostly in line with expected power values, rather than encouraging everyone to use their favorite rolling method then complaining when things are too easy/hard.
The way my group did it, we gave everyone one shot at rolling and if you didn't like the result you could point-buy as normal. Seemed to work out well.
The way my group did it, we gave everyone one shot at rolling and if you didn't like the result you could point-buy as normal. Seemed to work out well.