Why were European people superstitious?

Because they were human!

Superstition is common to all humans everywhere. People invent and repeat superstitions when they lack scientific evidence. Science develops slowly with a lot of trial and error along the way. The farther back you go in time, the more superstition you find on any continent. Superstition is not always wrong. It is bad luck to walk under a ladder, as can be proved by looking at work-place injury statistics.