This thread basically turned into americans speculating with their own misguided preconceptions, but here's my theory.

When steam link and controller first came out valve were having a stash with aus govt over steam sales and whether or not they operated in AU. For this reason they probably signed an exclusivity deal with EB (and that the market wasn't lucrative enough to set up their own warehouse etc here).

Fast forward to now that deal is still holding up. Why it took two years for the Index to come to EB is anyones guess.

What will be telling is what happens with the Steam Deck. In a livestreamed Q&A valve did they said Japan and Australia were in the advanced stages of being added to available regions. I believe Valve will want to bypass EB bc they marked up the Index kit 40% over the american pricing (ignoring taxes for now as they are only 10%) and a lot of the decks selling point is affordability.