Many of the world's strongest standards – including those of the internet – were standardized from work that first proved itself outside "standards bodies", rather than emerged from them. Standards bodies are better at codifying practices proven to work than bootstrapping new approaches, and quickly become highly-political impediments to innovation as the stakes grow & entrenched interests arrive.
Also, despite the ideals & good work of early ActivityPub work, in practice its server-centricity has already sent it many evolutionary steps down towards the same semi-feudal architecture as email & HTTP, where your identity, content-policies, & even privacy are at the mercy of your "home server" (feudal lord).
Even the real option to freely choose a new liege (at some serious switching costs) doesn't promptly or fully address this weakness, and the theoretical potential for practices to be radically different faces a giant "architecture tax" from the installed base, & costs/expectations of backward-compatibility.
Users of ActivityPub have already self-selected for those who are OK with such "home server" dependencies. Why, they find them "cozy" & even have (well-reasoned!) apologetics describing ways in which this user-homeserver co-dependency is good – for their needs.
So why should people with a very different vision, informed by both the limits of the giant proprietary platforms and of ActivityPub, fight a slow uphill slog in that community, as opposed to pursing a "green field" (or you might even say "blue sky") effort with fewer constraints? Appropriate formats & conventions from prior work like ActivityPub can still be reused, whenever helpful.