Why are developers supporting the Fediverse even though the Fediverse "only" has 10 million accounts?
Because the Fediverse offers something developers crave: stability.
As Twitter already demonstrated, they can remove API access from developers for many bullshit reasons -- with no explanation.
No company should ever depend on Twitter's API -- or any Big Social API for that matter.
But ActivityPub is a W3C-backed web standard. It is an open protocol. Unlike Twitter, it is more trustworthy.