One thing I've noticed in Marvel comics since at least the '80s: whenever there's a problem involving magic, and the writer doesn't want Dr. Strange to show up and fix it, there has to be an excuse given for why he's unable to show up.
Well, not exactly an excuse. Usually they just say he's "unavailable" or they "can't find him."
And there was a recent Fantastic Four where Reed calls Dr. Strange, only to be told that "Stephen is needed elsewhere." Because the writer doesn't always know exactly what Strange is going to be doing in other books.
Part of the point is not to have Strange show up and get beaten by the magical threat of the week - he's supposed to be the most powerful sorcerer, and he looks bad when he gets taken down by every penny-ante magical creature. In Infinity War, it takes four mystics to do what Strange could do on his own, emphasizing just how powerful he's supposed to be.