I've read that the discovery of CBR was accidental. Would it be possible for the discovery of CBR to be delayed until the modern-day?
What about just killing the ability to discover it? Is there some way to make astronomy massively uninteresting for the populace (fewer of whom would be astronomers) and/or governments (minimal funds also equals minimal astronomers)? Maybe other fields dominate to the extent that the ones without any connections to them suffer from a lack of numbers and funds?
Maybe a nuclear war. Humanity's been driven to go to the stars since the beginning of recorded history.
Which is why we're doing so much in space right now and NASA has more funds than ever.