Weston has better support for some things embedded developers care about, e.g. fallback codepaths for multi-plane buffer formats doing the blend in a shader, support for certain deprecated protocols (e.g. ivi-shell) and partial support for using hardware overlay planes for power-conscious compositing. It also doesn't have any of the Gnome dependencies. These are blanket statements; the devil is very often deep in the details.

Depending on the OEM you work for and how vertically integrated your organization is, you may also have a large and complex supply chain catering towards your products, with complicated RASIC matrices for who supplies you want and vendors held to KPIs for their BSP and their hardware. Due to various factors Weston may be what they already have patches in hand for, e.g. for proprietary buffer integration, etc.

(I work on KDE Plasma, including its Wayland support, and am a system architect for a large automotive OEM making Wayland-based infotainment systems, neither of which use Weston however.)