The truth is that the GNOME community probably has the most designers and UX-focused members in its community.
The problem isn’t lack of designers.
The problem isn’t lack of designers.
Having an entire CSS engine at their disposal certainly does help the designers design without needing to know programming.
There are conflicting design goals throughout the UI. All could be good/great with a cohesive system, but there is none.
To answer your question, the same people that wanted the system to be a Tier 1 mobile/tablet UI.