More than anything, the Spanish were seeking wealth.
Christopher Columbus himself thought that he had reached India: he wanted the wealth from the trade with Indians.
In the beginning, the Spanish hoped for Gold and Silver, later also for other products that could be made cheaper in the New World.
In short: the Spanish were not interested in Mexico but in what the ships could bring from Mexico (again, mostly gold).
Some, very few Spaniards, who settled in the New World were too unhappy in Europe and hoped for a better life.