Not Invented Here
One of the noticeable impediments in free/libre software is the Not Invented Here (NIH) syndrome. Others refer to this as development silos. The tendency to not use ideas from other software projects. Considering that all free/libre code is reusable and the philosophy of free/libre software encourages reusing code, why NIH exists is mostly a human nature ego challenge.
Humans tend only to follow social groups that confirm their own beliefs and perceptions. Software design is no exception. Free/libre desktop environments are particularly prone to NIH because design is a significant portion of the development.
People tend to be afraid of being called copy cats. Or to admit they did not think of something first. Yet outside software development, designers routinely grab and adopt the ideas of other people. The auto industry is an example. While style differs among the auto manufacturers, the basics of design are much the same. New users know they can find speedometers, gauges, and warning lights in the dashboard.
Human existence much depends upon sharing ideas.
This is not the case with free/libre desktop design. The existence of any particular feature in one environment is no guarantee of the same or similar feature in other desktops.