Question

Why were lanthanoids and actinoids not placed in mendeleevs periodic table?

Answer 1

Answer:

Mendeleev arranged the elements in order of increasing atomic mass, the properties where repeated. ... In devising his table, Mendeleev did not conform completely to the order of atomic mass. He swapped some elements around. (We now know that the elements in the periodic table are not all in atomic mass order.

Answer 2

He knew all lanthanides except prometium and two actinides: uranium and thorium.

He included them in his periodic system. He realized that the chemical properties of La, Ce,  Lu are so similar that he share a common cell. He put, however, thorium in the IVb and uranium into the VIb column of the transition metals according to their highest valences.