With a machete, why is a diagonal cut more effective than a right angle one?

The flexural rigidity of a vertical beam will be greater with a diagonal cut. This allows more of the force to be channeled into cutting the target as opposed to bending the target.

The determinants of flexural rigidity affected by a diagonal as opposed to a horizontal cut are the cross section of the beam and distance from the supported end of the beam. The elasticity of the material is measured by Young's modulus, but that's not at issue here as it applies to the material itself, not to its positioning or structure.

A diagonal cut effectively increases the beam's cross section, which increases rigidity.

A diagonal cut transmits more force into the ground (as Jerry Shirmer and CoilKid noted in their comments), which effectively shortens the distance to the supported end of the beam.

For a beam in tension or compression (as one would expect of a vertical beam), the axial stiffness is:

k = A * E / L

where:

A is cross section area,

E is Young's modulus or the modulus of elasticity (Bending stiffness is also a function of the modulus of elasticity, cross section, and length: https://en.wikipedia.org/wiki/Bending_stiffness),

L is length (effectively length to the ground).

The diagonal cut effectively increases A and decreases L, resulting in greater k and more force put into cutting, less into deforming the target.

Simple answers:

When you cut diagonally, less material is being moved aside during a given bit of time. The time of the cut is longer, the force the same, therefore you are applying the same force to less material in a given moment. Therefore it's easier to cut the longer arc, even though the force is unchanged.

Also, gravity is assisting your diagonal blow, increasing the force of your blow, while a horizontal blow is unaffected by gravity.

Also, your musculature is better suited to a 45 deg. angle than to a horizontal.