Xbox addons are limited to what’s allowed in the marketplace only. PC addons can be installed from external sources, the marketplace is pretty hard to get access to sell addons on so it’s avoided where possible.
A lot of products require frequent updates as new updates for the sim come out, the marketplace delays these updates by up to a month, by which time the sim is already about to get its next update so it’s not worth the time to even bother with it.
Also, the SDK that addons are developed with is relatively tricky to figure out how to use, there’s a years worth of experience with making addons work on PC but only a couple months on Xbox, I am sure a lot of these addons will come to Xbox at some point but it will be limited for a while in terms of what products come to Xbox, there just isn’t the resources there for it yet.