I have my own theory as to why - the Digital Markets Act in the EU: https://commission.europa.eu/strategy-and-policy/priorities-2019-2024/europe-fit-digital-age/digital-markets-act-ensuring-fair-and-open-digital-markets_en

Examples of the “do’s” - Gatekeeper platforms will have to:

• allow third parties to inter-operate with the gatekeeper’s own services in certain specific situations
• allow their business users to access the data that they generate in their use of the gatekeeper’s platform
• provide companies advertising on their platform with the tools and information necessary for advertisers and publishers to carry out their own independent verification of their advertisements hosted by the gatekeeper
• allow their business users to promote their offer and conclude contracts with their customers outside the gatekeeper’s platform

The interoperability is the big one. Since Threads isn't currently connected to the wider fediverse, that's probably one reason why they're not in the EU yet - because it's currently in violation of the Digital Markets Act. This also means that fears of "Embrace, Extend, Extinguish" are likely overblown. Extending ActivityPub without working in tandem with the standards authorities to ensure interoperability means that they'd be (once again) in violation of the DMA.

I'm not saying Facebook is innocent. People actively say Facebook is malicious (not without cause, mind), but really - like all corporations - Facebook is self-serving. Facebook will always do the thing that is in their own best interest; they're sociopathic but not evil for evils' sake. And the DMA gives clear evidence that EEE is not in Facebook's best interest.

Let me quickly quote selected sections of the act itself:

The ability of end users to acquire content, subscriptions, features or other items outside the core platform services of the gatekeeper should not be undermined or restricted. In particular, a situation should be avoided whereby gatekeepers restrict end users from access to, and use of, such services via a software application running on their core platform service. For example, subscribers to online content purchased outside a software application, software application store or virtual assistant should not be prevented from accessing such online content on a software application on the core platform service of the gatekeeper simply because it was purchased outside such software application, software application store or virtual assistant.

...

The lack of interoperability allows gatekeepers that provide number-independent interpersonal communications services to benefit from strong network effects, which contributes to the weakening of contestability. Furthermore, regardless of whether end users ‘multi-home’, gatekeepers often provide number-independent interpersonal communications services as part of their platform ecosystem, and this further exacerbates entry barriers for alternative providers of such services and increases costs for end users to switch. Without prejudice to Directive (EU) 2018/1972 of the European Parliament and of the Council (14) and, in particular, the conditions and procedures laid down in Article 61 thereof, gatekeepers should therefore ensure, free of charge and upon request, interoperability with certain basic functionalities of their number-independent interpersonal communications services that they provide to their own end users, to third-party providers of such services.

Gatekeepers should ensure interoperability for third-party providers of number-independent interpersonal communications services that offer or intend to offer their number-independent interpersonal communications services to end users and business users in the Union. To facilitate the practical implementation of such interoperability, the gatekeeper concerned should be required to publish a reference offer laying down the technical details and general terms and conditions of interoperability with its number-independent interpersonal communications services. It should be possible for the Commission, if applicable, to consult the Body of European Regulators for Electronic Communications, in order to determine whether the technical details and the general terms and conditions published in the reference offer that the gatekeeper intends to implement or has implemented ensures compliance with this obligation.

Simply:

• Content should not be restricted to just your site

• You need to allow third-party apps

• You must publish your API publicly

Failure to comply causes:

• Initial fine of up to 10% of the company’s total worldwide annual turnover (or up to 20% in the event of repeated infringements)

• Daily fine of up to 5% of the average daily turnover

• Systemic infringements can cause the EU to break up the company entirely

The law has teeth. It started to take effect in May, but there's a period of time before compliance is expected (likely we won't see much compliance until 2024). The route of least resistance for Facebook is to adopt ActivityPub and allow third-party ActivityPub apps like Fedilab to interface with Threads. They can't make changes to the protocol because then they won't be compliant with the Digital Markets Act. I wouldn't be surprised if Instagram and maybe even Facebook itself start to federate as well.