Mechanically, I am sure it is that way just because it was the default farm layout, and it was tailored to actively encourage you to go out and fish at any/all the other locales in town, with other farm layouts to provide better home fishing options in exchange for space/etc.
As for a story reason though, hmm.
I think the main thing with the default layout is:
• It is big and wide open
• It is land-locked
With the riverside and beach farms, the water source carries on from elsewhere and generally continues onward too, so trash is less likely to accumulate en-masse in these areas. And while the water in the forest map is also land-locked, it is surrounded by trees and other cover on all sides, and may perhaps be influenced by a touch of forest magic too.
Grandpa has been gone for some fair time after you open the letter and go to the valley, so it took time for it to get to the overgrown state it is in. When the land is so flat and otherwise exposed, it is easy for all sorts of things to blow through on a windy day. I live in a similarly exposed area, with just some trees and shrubs around my house for cover. When it is windy on trash-pick up day, it can be problematic when there is lots of lightweight trash in the can and it can't stand up to those winds - it can get knocked over and trash will just blow around into wherever it can snag - usually my shrubs and trees. Sometimes I find other people's trash around here after windy days too. Perhaps on the default layout, there is a similar issue with a lot of trash having accumulated in the water just due to wind blowing it around until it catches on the weeds at the sides of the pond or otherwise gets wet and sinks.
That would be my interpretation, anyways.
