This answer might surprise you, but the reason why 1366x768 is the most common laptop resolution is because a dominant notebook LCD-panel manufacturer called AU Optronics (AUO) makes vast majority of their panels in 1366x768, regardless of the actual size. Many of their 11'6", 12.5", 13.3", 14", 15.6" panels only come in 1366x768.
These panels can be found in sub $500-$600 laptops, which accounts for majority of laptop sales. Hence 1366x768 being the most common screen resolution. (Yes, even most of 11" MacBook Air panels are supplied by AUO.)
Other than high-resolution and high-priced panels, lower-to-mid tier panels are dominated by AUO.
This is AUO notebook panel line. Note the prevalence of 1366x768.
AUO is the third largest LCD panel makers after Samsung and LG, but the latter two are absent from inexpensive laptop display market, as seen by the first link.