Why can't I preorder some games on the xbox live store.

why I can't get the game of unbound in my Xbox X I preordered and Paid the game

Unfortunately Microsoft only allows you to pre-order games that have some kind of pre-loading.

Games that aren't in manufacture or close to that point aren't open to pre-orders because the Xbox has nothing to download.

Either it is a fault in the system that Microsoft can't process a pre-order without something being downloaded or they did it on purpose to stop some getting confused why they didn't get anything for their pre-order.

It all depends tbh,some go up early and some nearer release date,its upto the publisher as to when/if a game is made available,so there's no definitive answer,sorry.you could always tweet them,or post on the official ubi forum and ask,maybe you could get
a date there

Some people can't make it to a store (remote areas etc) .... but in general, if you can make it to a store, you'll be able to buy withotu preordering. infact if you dont preorder you can go to any store to get it and avoid lineups. its only EB/gamestop that
occasionally run out of stock (intentionally) .....you never see a game sell out at other stores on launch day.

Are you asking this because of the add-ons and exclusive that preorders come with? If so, you can't preorder a digital game but you can get all the free add-ons in the Xbox Store the day the game is released. The only exclusive you can't get for free are
the ones that retail store may have solely for those who purchase from them.