Well one reason would be to get a classic old bobber look to the bike. Somethin' like this one:
The bike could possibly handle better too. It wouldn't take as much effort to lean the bike with a skinnier back tire. You would also have a reduction in weight.
But like Greybeard said, the skinnier Bonneville tire would look out of place since the rest of the rearend is built to take a much wider tire.
