[Nickj]

It's the current 'style', Ducati have to keep up with whatever the opinion polls and design studios say is the trend to follow to catch their perceived market demographic.
It's why everyone's bikes look like everyone else's.
The trends often aren't driven by a huge cross section of the end market but more by what the factory designers see coming out of the trendy big name design houses... which is then what the new wave of design teams and designers see as the way to go. It's also perceived as a safe low risk path to follow by management which is ever more risk averse.
You can see it all across the consumer market in almost every kind of product you can buy.
It's all a bit self fulfilling and takes a brave team to buck the trend and do something different to what is the current design trend of the day dictates ..... like the original monster concept.

Some companies selling their niche keep the design cues ... Triumph make engines that externally are kind of similar to the old twins, the new Nortons probably will as they're an off shoot of RE which trades mercilessly on it's heritage look (but they're run and massively influenced by a 'brit' bike enthusiast). HD just keep the same as they are so heavily subsidised they can't be arsed to change much and actively squash dev work like Buell's.

It's the corporate times we live in.