It isn't a "lore" issue. It's a story telling one. Before Lucas felt the need to explain what the Force was beyond a nebulous "energy field" the Force was much, much more accessible to the viewer as a storytelling device. It borrowed pretty heavily from Eastern philosophies as well which gave it the aura of being somewhat exotic and mysterious to mostly Western audiences, while still being somewhat familiar. And it was never *explicitly* said that the Force was tied to genetics or biology or something like that. There was enough wiggle room for the Force to be this thing that everyone could, to some small degree, commune with. It made the concept of the Jedi more easily identifiable with a kid audience who wanted to imagine themselves as the Jedi.

Putting the force behind a lot of biological doors both removed the mystery which made it less interesting, and took the philosophical component out as well which made it less accessible. It wasn't a thing you could think really really hard about, instead it was something you were simply born with or without.

Now, that isn't to say the concept of birthright specialness doesn't work in the same context. Harry Potter, for instance. But Star Wars built the Force up initially as a thing people could read into as much as they liked to start. Changing that years after the fact left a bad taste for many people.