Why does it generally take more enthalpy to ignite a solid than a gas or liquid?

It is due to the nature of the reactants

Explanation:

To ignite a solid, we require more heat component compared to liquids and gases. For ignition to occur, oxygen gas combines with a reactant in most cases.

Some factors affect the rate rate at which a chemical proceeds. One of the factors is the nature of reactants.

The solid phase is very slow while the gaseous phase is rapid and fast.

            solid phase < liquid phase <  gas phase

Gases are free and the molecules move in all direction. They easily combine and react very fast.