That is not dead which can eternal lie circumspect,
And with strange aeons even threads may resurrect.
3d6: you know, they were a lot more limited by what kinds of dice they had--'little brown book D&D' used 6-siders, and they didn't have computers to generate random variables that couldn't be made by dice. Expected value of 3d6 is 10.5, and that was close enough to 10, which is a nice round number and also evokes the average 100 IQ.
Now technically IQs are normalized to have a 100 average and 15 SD, and 3d6 has a standard deviation of 2.96, not 1.5, so the distributions aren't the same--3d6 varies too much to be IQ/10. You could go for 4d4 if you wanted an average of 10 (with a more appropriate SD of 2.24), and 5d3 is still too variable at SD 1.8 and 6d2+1 too un-variable at SD 1.2...but nobody thought that hard about it, and before computers they would have had to spend a lot of time messing around with pen and paper and/or go to the library to look up a statistics book (and who knew what the Lake Geneva public library had). The 1e DMG has a picture of a bell curve, so probably Gygax heard somewhere it was close to that, thought 'good enough for a game' and left it at that.
Dude just wanted a 'smarts' stat to go with the 'strength' stat.