Combine color and stroke in one palette
Older versions of ai had color and stroke in one palette... since in almost 100% of the cases when you draw an object you will need to modify both. This could be be an option under both current palettes.
Still, why not to just dock one panel to another?
This is exactly why Properties panel was introduced, but failed to achieve.
I’d suggest to use Appearance panel instead, which allow to access both stroke and color panels in the same way.
But answering the question, I’d say 'to avoid cluttering'. Adding a selector can lead to adding some other UI elements from a Stroke, and in the end we get another panel that allows to set some things, but not others, and two intermixed panels instead of clearly separated ones.
Jan Vork commented
When tweaking the Stroke of an Object, it's nice to have all properties in one panel. Why not add a Color selector to the Stroke panel?
I kinda support this but I would like it to be a part of more broad improvement. I can dock Stroke in Color, right, but I can't land these two combined to the side with other palettes, because this hybrid will take the whole separate column. Please, think about improving this.