The short answer is - it doesn’t look right to me - which frankly is enough of a reason! But why doesn’t it look right?
The problem is that color and black-and-white images appear to be from different times or even different worlds and sometimes both. They’re so different from one another that presenting them together “jars” and breaks the continuity of the presentation. The viewer ends up paying more attention to the way the images are presented and attention is deflected away from their content.
In this shot the cool guy is clearly the subject of the image. The bike is incidental.
When I see a monochrome image I think about it in a different way. It’s less about the “how” (it’s easy now to create colour and/or mono from the same image) and more about the “why”. What is the image maker trying to convey by choosing to display an image in monochrome. What began as a necessity, then accepted as normal has now become a matter of choice. That choice means something, creatively, and in my view should be regarded separately and differently.
In this edit the bike detracts from the focus on the subject with it’s attention grabbing colour
If that’s true of course then one could argue that an imagemaker should already know whether an image will be colour or mono before the image is captured. I confess that there are however plenty of times when I look at a photo in the editing suite and try it in both colour and mono before deciding! Hey - nobody said I was perfect………