Here's why:
* Adjectives in French agree in gender and number with the noun they modify.
* "Intelligent" is an adjective.
* Most adjectives ending in "-ent" are masculine.
However, there's a catch! The feminine form of "intelligent" is "intelligente". So, you'd use "intelligente" when talking about a woman or a female entity:
* Un homme intelligent (a intelligent man)
* Une femme intelligente (an intelligent woman)
Keep in mind that the rules for gender agreement in French can be complex, so it's always helpful to double-check the correct form.