I agree that it should have always been an option but I'd imagine that it was largely down to their legacy systems.
To be fair, none of the console stores are quite what they should be as there are still issues with buying some legacy games on the Series X in that you can select a game on the console but then it tells you that it's not available and that you need to buy it via the website. That's been there since the start of the last generation and it is stupid to have games listed on the console which takes you to a page that tells you to just buy it on the web store.
I would assume that was down to legacy systems too, hopefully they'll all end up where they should be at some point but there's always going where they spend their time and money, particularly on legacy systems that may not be perfect but they sort of be OK.