Historically, triangular shaped flags in South Asia were very common, since it was compact in size so the flag furled even with the lowest wind, thus making it visible over long distances. The flags of almost all states in South Asia were once triangular. Nepal has simply maintained its ancient tradition, while every other state has adopted a rectangular or square version in the European vexillological tradition.

en.wikipedia.org/wiki/Flag_of_Nepal#Aspect_ratio