Sorry, I’m just now getting to this but yes I can explain.
It’s got nothing to do with Walt and everything to do with how Disneyland itself is laid out. Unlike MK, the river is a part of multiple different lands that were all designed to interact with it. Frontierland, New Orleans Square, and Adventureland all three have a view of it and interact with it. All three lands were built with it in mind and it functions as a critical part of their visual identity.
This is not so in Magic Kingdom. In Magic Kingdom, the river is only impactful to Frontierland a part of Liberty Square (but not even that in it’s entirety). Instead of being a show piece for the entire west side of the park that that entire side of the park is built around, it’s tucked away on it’s own.
In the design of the park, they made it not be a focal point but rather just an element of one corner of the park. They did not build that side of the park around the river and it plays no role in any land other than the land it solely resides in. It is not a visual tying together of multiple places, it is one singular place.
The entire left side of Disneyland relies on the River. The park is designed in such a way that it is irreplaceable. That simply is not the case for MK. It’s loss there has no impact on anything other than Frontierland itself. From any other land it may as well have mot been there at all. I don’t know why they chose to do it that way, but they did. It’s one of a few layout choices made for Magic Kingdom that I think contributes to it not being as tight as Disneyland.