Full Idea
Leibniz believes in monads because it would be contrary to reason or divine wisdom if everything was compounds, down to infinity; there must be ultimate unified building-blocks; they cannot be material, for material things lack genuine unity.
Gist of Idea
Leibniz proposes monads, since there must be basic things, which are immaterial in order to have unity
Source
report of Gottfried Leibniz (works [1690]) by Nicholas Jolley - Leibniz Ch.3
Book Reference
Jolley,Nicholas: 'Leibniz' [Routledge 2005], p.74
A Reaction
It is hard to discern the basis for the claim that only immaterial things can have unity. The Greeks proposed atoms, and we have no reason to think that electrons lack unity.