Why did Mendeleev’s predecessors fail to classify all chemical elements?

The knowledge about atoms, at that time, was very insignificant, one might even say – incomplete. Few people imagined that there is an atom and all its properties. It was Dmitry Ivanovich Mendeleev who was the first, at the time, as the legend says, in a dream, to see some patterns of atoms, which allowed him to combine them for classification and create his famous Mendeleev’s Table.