D3’s default path interpolation is the same as its string interpolation: it finds numbers embedded in strings, and interpolates those numbers. So, the default behavior when interpolating two paths is like this:
M x0, y0 L x1, y1 L x2, y2 L x3, y3 ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ M x0, y1 L x1, y2 L x2, y3 L x3, y4
For example, the first point ⟨x0,y0⟩ is interpolated to ⟨x0,y1⟩. Since x0 is the same, all you see are the y-values changing (see example)—you don't see the path slide to the left as intended.
What you want to happen here is something like this: