I don't know the specific design criteria but from my perspective it was for ease of installation.
For me the install of the sight gauge plates and decal/placard came in the finishing touches of the build process. If I had put it in when the wings were built then I would have probably gone to great lengths to keep it from getting bunged up during the built process etc. and it would have taken extra work to do that.
But hey, that's just my approach.