for one reason, space is almost a vacuum, so that molecules erm are few and far between, and one thing about chemistry it is really the science of not particularly molecules but molecules that react with one another, but here once one has got a molecule in space it doesn't actually meet another one for a very long time, so even a molecule that is reactive and which may only last for maybe a microsecond in the laboratory, interstellar space it may last for a thousand years.