According to usually well-informed FIFA EA Sports forum user Davide1774, the reason for FIFA not featuring the latest Nike boots is pretty simple - EA wants more advertising in the game (there are many Adidas adverts in FIFA traditionally, e.g. in the FIFA 19 The Journey Mode).
"Nike has asked for more EA advertising in the game. They still have not found an agreement, which they will find as soon as possible. We just have to wait and told me that the EA is sorry for all this!"
let's just hope, that the new packs will be included really soon.