Unity's coordinate system is Y up. Using worldPos.yz for uvs on the X facing plane would result in that plane's texture being rotated 90 degrees. Using worldPos.zy keeps the alignment the same as the Z facing plane's worldPos.xy. While keeping the alignment the same may not be needed for a lot of textures you might use with triplanar mapping, it is important for some. The brick texture used in the asymmetric blend for example, the X facing plane's bricks would line up with the top instead of the other side.

If you're using this technique in an engine that uses Z up, worldPos.yz would be more appropriate.