The reason why #Threads and #Mastodon are incompatible is that Mastodon is a community, and Threads is a market. That's it.
It's not big Fed vs small Fedi. It's not platforms vs protocols.
It's communities vs markets.
Communities are supportive and markets are extractive. Communities answer to those who must live with decisions made. Markets answer to money.
And wouldn't you know it, those contrasting forces influence people's behaviors, goals, and motivations for engaging with others.