i found this explanation. Hope it makes sense.
Think of it this way. A developmental editor is a coach. A beta reader is a fan.
In baseball, a hitting coach can analyze your swing, telling you your balance and timing are off, you're dropping your hands, you're rolling your wrists, you're loading too soon, etc. A fan can tell you you're not hitting the ball well. They might sort of know why (you're swinging at too many pitches out of the strike zone), but they won't be able to explain it and help you improve like a coach would.
From my own experience, the editor I used helped me see where parts weren't working and more importantly how to fix things. She was looking from a technical point of view. Beta Readers are more like true readers for pleasure I guess.