European countries often have stricter safety regulations than the US. I wonder if there was a time when the 18's, with their smaller wheel/leg base, didn't meet a regulation for stability or something.
If this was the case, the engineers at Weber would have simply gone to the parts bin: legs from a thrifty kettle, a triangle from a 22, and a bent piece of flat stock bolted to the front leg. Bam! a 47cm kettle, ready for export to the European markets.
All just speculation of course.