I'll add another guess. A resource crunch, i.e. we simply use up the required resources before we figure out how to explore our planetary neighbourhood. e.g Wh

I'll add another guess. A resource crunch, i.e. we simply use up the required resources before we figure out how to explore our planetary neighbourhood. e.g When we figure out that a helium engine is possible, very efficient and very safe, we no longer have sufficient quantities left.