If our crystal was a perfect sphere, then this wouldn’t matter: All diffracted beams will have to travel through the same length through the crystal (assuming that diffraction ‘happens’ at the centre of the crystal).
But real crystals aren’t perfect spheres, and the more anisotropic they are, the bigger the problem will be: in certain directions, the diffracted beams will have to travel through a lot more crystal than in other directions.
Why does this matter? Remember: the position of the diffracted beams is defined only by the unit cell dimensions (a, b, c, α, β γ). The intensities of the diffracted beams are defined by the contents of the unit cell, i.e. the bit we are interested in: the structure!