Why are lanthanoids and actinoids shown below the main periodic table?

The lanthanoid and actinoid series are placed below the main body of the periodic table primarily for practical display purposes rather than for any underlying scientific necessity.1

Including these 28 elements within the main grid would result in a very wide table, making it difficult to read and manage. By extracting the 14 lanthanoids and 14 actinoids into a separate double row at the bottom, the table is compressed into a more compact and convenient format.21

While this arrangement is a matter of layout, it is worth noting that these elements are categorized as inner transition series because they involve the filling of f orbitals. Because their chemical properties often closely resemble those of the elements at the start of their respective series, this grouping also helps maintain the logical organization of the periodic system while keeping the overall structure visually accessible.342