I think this may primarily be a consequence of three things:

1. The head being split into two sections along the cam centerlines.

2. The symmetric crossflow design with the plug in the center

3. 4 valves per cylinder

Because of these two things, they had to tilt the valve line out of a vertical position because there wasn't any room for the tops of the valves in that center region in the top cyl head section as the spark plug well was there, but the bottom of the valves did have to be in the combustion chamber.

There weren't a lot of things to change in the combustion chamber while fitting 4 valves in & trying to maintain good cylindrical symmetry with a minimum of discontinuities/obstructions. The end result was that the axis of the valves are angled at about 30° to the axis of the piston with the bottom quadrant of the valves sitting pretty close to lower surface of the lower head section & the top of the piston.