Because they signed him to promote their whole line of clothes, not just tennis things, which is a very small segment of their catalog. Thus, they are not fully interested in promoting every tennis outfit Federer wears.
I don't understand why this pretty simple concept is so difficult for Fed/Nike fans on this board to understand or accept - at least those always complaining about the distribution levels of Federer's tennis kits.
It is not difficult to understand (even for them), but it is a thing they can blast Uniqlo/Federer for, and they are doing it for it.
You just need to look into the behaviour of some of them to know it: like the guy who cries that there is no Fed merchandise, that his local Uniqlo doesn't have "anything", that he doesn't like the colours and designs, that they don't have XXXL T-shirts for him to wear etc. It is a circus from the Nike fanboys that are angry that Federer left.
There is a very small minority of fans that would like to have to opportunity to buy some of his Uniqlo outfits, but they don't approach the matter like the above group, and yes, it would have been nice if Uniqlo was selling his outfits even in limited quantities, as there were quite a few that were very good. I think that if they didn't want to overstock they could have just continued to do what they did with his Wimbledon 2018 kit: preorder, so that every single outfit is sold for sure.
