It's all down to numbers of potential customers. In the UK we may have a reasonably healthy rental scene but people who actually buy and own their kit are comparatively few, only a few thousand.
In the US they have many more tournament teams and vast numbers of rec ballers with their own kit, so it makes sense for the manufacturers to concentrate all their advertising and sponsorship on that market.
It costs the same to fully sponsor a 7-man UK team as it does to sponsor a 7-man US team, but you are getting to many times the number of potential customers.