This is a strange thing, but it is usually explained as dialectal influence and it seems to be the correct explanation for catholic Saint named Aloisius.
However, names spelled
Alois existed long before that and the most satisfactory explanation for the earlier appearances of the name is a Germanic one, a dithematic name composed of elements AL "all" and WIS "knowledgeable" (cf. wisdom). An Alwis occurs in the poetic
Edda (the unfortunate guy is tricked by
Odin in a riddle contest to stay until day dawn and is turned into stone by the sunlight; but the contest itself is a nice crash course on Germanic mythology).