Because 130/90/16 is the stock size for my '00 XL Custom. Going a little wider (140) will be easier to achieve with the shorter tire. It is possible to get the 140/90/16 under there, but you have to be careful with the bolts and wires inside the fender. Using a shorter tire (even that little bit) makes it an easier change.
If I have to...I will use a 140/90/16, but for now...searching for a 140/80/16. Thanks.