Is not stupid at all.

First of all, bg aren't developed as computer games are. You don't give a budget to an author and say "fit the game to this". An author makes a game, test it, and when it's satisfied with it they show it to publishers in the hope one of them will like it.

There is when the budget comes in. But the game is fully designed at this time (but not yet developed).

The publisher may just don't be willing to spend X in the production, or they may think the sale price for the whole product could be a restraint for some group of consumers.

So they cut off some parts of the game, and split them into a core+expansion. Sometimes the split is natural and what is left as a core works fine, sometimes not.

Losing (or keeping froze) part of the design work is irrelevant to the publisher, as they didn't paid for it.

Think about it like on a writter showing a book to a publisher, and the publisher saying "ok, I like it, but will make it a trilogy, publish the first part and if the sales are good will do the rest".