Probably just a lack of awareness. If you read the support page closely, Yubikey 5 is given as an example of a supported FIDO2 key, but I’d bet that the Titan works as well.

It’s kind of like how Google clearly states that you need to use Google Authenticator (tm) as 2FA for your Google account, but really any TOTP app will work.

It's because it doesn't implement FIDO2 but only the older U2F standard. It's a bit of a dated key and I wouldn't recommend it.

It works fine for me on GitHub, Microsoft, AWS and many others.