There were a lot of characters that needed to share screen time in Avengers: Infinity War. Apparently, at one point this meant keeping one important character off the screen for a long time. It seems that in one draft of Avengers: Infinity War, the moment where Captain America jumps in from off-screen to tackle Corvus Glaive and prevent him from killing Vision in Wakanda was actually the hero's first moment in the movie. According to the writers...
Generally, Stephen McFeely and Christopher Markus are heralded as some of the strongest people involved in the Marvel Cinematic Universe, being responsible for the two previous Captain America movies that are very well regarded. However, it seems even they are not above criticism, as some unnamed people, likely Marvel execs, thought the pair were insane for keeping Captain America out of the move for nearly its entire runtime. The writers don't reveal exactly where Steve Rogers apparently was during the entire run of the movie, beyond saying he was "underground." Perhaps the plan was to reveal that in Avengers 4.
It certainly would have been a gutsy move to keep Captain America out of Avengers: Infinity War until the end. If handled perfectly, it might have even worked. The roar from an excited theater crowd could have been something special on opening night if the anticipation had been built just right. However, Christopher Markus says specifically in the writer's commentary track of the new Avengers: Infinity War Blu-ray that it was not satisfying, which is likely why the decision was made to change things up.
Instead of coming from nowhere, Captain America arrives alongside Black Widow and Falcon and rescues Vision (and Scarlett Witch) in a much earlier scene. It's a moment that does feel satisfying and doesn't leave the audience wondering where Captain America is for the entire movie.
It feels like Markus and McFeely may have been going for a sort of Star Wars: The Force Awakens vibe with Cap playing the Luke Skywalker role. In that case, however, a larger part of the movie itself is about going in search of Luke, so it's expected that he won't be found until later in the story. Here, nobody would be looking for Captain America, there's too much else going on, and he would just appear from nowhere. It's not surprising the moment didn't feel satisfying.
Instead, we got a movie that it seems most would agree was quite satisfying. Now, we just need to wait and see what's next for the Avengers and what great moments are in store for them in Avengers 4.