It was more than a cameo. He’s in only one scene, but it’s a fairly pivotal scene.
One possible reason I can think of is that Russell didn’t want anybody coming into the theater expecting a “DeNiro movie.” But surely he had enough control over the advertising and press to keep that from happening anyway, and I don’t see how scrolling Bob’s name in the end credits would have changed anything in that regard.
And not just in this case, but other stars have appeared in movies without credit before. If it’s a surprise cameo that would amount to a spoiler if known in advance (Johnny Depp and Peter DeLuise in 21 Jump Street, for example) I can see the reasoning, but otherwise, what’s the point?