Why cannot the minimum order amount (or other settings) be changed in the shopping cart?

This can be if the shopping cart has been added to each page separately for a specific product/service, and not in the Header or Footer of the website.

If different shopping carts have been added to different project's pages, you need to apply changes to each page's settings and content — changes in one block cannot be applied to other blocks with the shopping cart on the website.

However, if the user adds a product to the shopping cart and jumps to another page — the product added is saved, but the settings automatically change to those that are relevant for the current page.

You can read more about how to work with the shopping cart in this guide.

https://help.tilda.cc/online-store-payments/cart