A question for all who have mail-in voting...

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9. It comes down to law, secrecy of the ballot, and logistics.

It was on the outside in 2020 also, and before that I'm not sure because the caucus results were used by the party instead.

To answer your question, it's required to be on the envelope in law at the request of the party. It used to be under a flap, but then a huge number of people forgot to sign it, so that was changed years ago. Then, once your county receives your ballot, they have to sort by party declaration, because if someone marks R on the envelope but a D on the ballot, the vote can't be counted. So, if it were on the inside, you'd be able to match the identity of the person to their vote (not just which party they identify with), thereby eliminating secrecy of the ballot (another legal requirement).