Good question. They did a similar thing in their autos back in 2001 and went from a 5w-30 to a 5w-20 weight.
I don't know the answer for sure, but my *guess* would be that it has to do with meeting emissions and increasing MPG ratings.
When you think about how a multi-viscosity oil is made, a 10w-30 is actually superior to a 10w-40 with regards to it's lubrication properties, because it will have less VI polymers in it, which means it has more oil and less additive. It will also not break down as fast, since th VI additive is the first thing that breaks down. So less VI will be present in the oil to begin with, so it won't break down (percentage wise) as fast.
Auto manufactures stopped using 10w-40 oil for this very reason about 10 or 15 years ago. They found the large amount of VI needed to make it was causing it to break down way too fast.
The bad news is that your choices on proper oil for the GL1800 have now become even more limited if you want to stick with the Honda recommendation. It is pretty hard to even find the Honda GN4 10w-30 oil. None of the dealers around here even have it in stock. And almost all 10w-30 automotive oil has friction modifiers in it, and is labeled as energy conserving and shouldn't be used in a motorcycle due to problem the friction modifiers can cause with the clutch.