Why do people like Cinco de Mayo?

To clear the common confusion, It has nothing to do with the Independence the declared from Spain on September 16, 1810.

Cinco De Mayo is a Mexican Holiday that was created to celebrate the victory of the Mexican army against the French in the French Intervention in 1862. The Mexicans with an army of about two thousand defeated the French forces of six thousand. Although they did lose the war, they still celebrate the victory of this battle.

To Answer your question, it became popular in the late 20th century when Dos Equis Beer company encouraged Americans to drink their beer on the Fifth of May (Cinco de Mayo), to promote sales.