That's a specific case of the more broad rule of "small is agile, large is cumbersome". It also applies to private enterprise. A small company of 10 can do in a week what takes a company of 1000 a month to get approved.
The larger something is the more paperwork is involved, but also the more factors they have to consider. National government can't put in individual rules for every single water body in the country. They need general rules that cover broad categories. Such rules are a lot more complicated.
