As Sebastian Stan also noted to ET, while Bucky will reunite with Steve Rogers during Infinity War, because they’ve gone through different life experiences, their relationship isn’t necessarily the same as it was back when they were buddies in the 1930s and 40s.
So even though Bucky and Steve are still friends, perhaps the former will end up forming a unique fond with Falcon during the hectic events in Infinity War.
With a cast as large as Avengers: Infinity War’s, it’d be ridiculously difficult, if not downright impossible, to ensure that everyone receives equal screen time with one another.
So it’s to be expected that certain characters will be paired together for longer than they are with others, and in the case of Bucky Barnes, it appears that he’ll be hanging out with Falcon for much of the movie.