Dear Sir

I am not sure if the reasons are explicitly stated. but Vedas in general speify certain principles.

minimize material complexity focus on spirit and spiritual matters

Don’t affect other living entities and environment

spend less time and effort on other matters other than spirit

So based on these principles if we see, dhoti is  very easy to manufacture without complication of machines etc.. it does not affect rivers and animals as there are no dyes and chemicals

if one grows up by age or size or becomes thinner it is usable.

It is one size fits all solution..even when it is worn out it can be used as rags.

so it makes sense that vedic people who followed vedic principles might have adopted the simplest and most convenient solution of dhoti as dress code.

Apart from the above reasons, Veshti offers flexibility to work, especially the north india dhoti style ..pancha kaccham type..

Also white colour represent purity and may be useful for spiritual practices and pooja..

But those who are more learned and knowledgeable in scriptures can help us give an more authentic answer. Lets wait

thanks

regards