The main reason is having Energy to spend. In 5.2/5.3, most Windwalker monks were energy cappign with Ascension around the 9k mark. However with the 5.4 changes to Chi Brew, Chi Brew became the go-to talent for that tier due to the Chi generation and TeB generation. With that change, the amount of Haste needed to keep up at or near energy cap increased quite a bit. So to help compensate, Monks increased their haste to 10-12k.