Nepal is an outlier when it comes to flag shape, as it stands out from the norm (rather “two triangles stacked on top of each other”) since according to them: the flag represents Himalaya Mountains & both religions: Hinduism & Buddhism, also the red triangular flag has been a Hindu symbol of victory since the writing of Ramayana and Mahabharata.
That is in contrast to most countries flags as a majority are rectangular in their shape (no matter where from USA, UK, Canada, Japan, Germany, Poland, etc.). I mean, why are most flags rectangle by default rather than being unique on using another shape? I mean, Nepal is the only country where a flag is designed from another shape that differs from a rectangle.
Maybe another thing worth considering is that rectangular flags are just bigger and easier to see than other shapes.
Also, maybe it just became “normal” to have square flags. The Romans conquered most of Europe, and they flew rectangular banners from their standards. Following the fall of the empire, the different parts of Europe were at war with each other for one and a half thousand years. I suspect all having had this original template, then the subsequent fighting / conquering / reconquering / reconquering, probably lead to this shape becoming normalised.