As given in the Wikipedia page,
An azeotrope (/əˈziːəˌtroʊp/) or a constant boiling point mixture is a mixture of two or more liquids whose proportions cannot be altered or changed by simple distillation.
It further goes on to give the reason;
This happens because when an azeotrope is boiled, the vapour has the same proportions of constituents as the unboiled mixture.
But why? Why do they have this special property? Why don't substances normally have the same composition in solid and liquid phase?