Why doesn't Brazil speak Spanish?

This is a question my sister wants to know. So of course Brazil was colonized by Portugal and therefore they speak Portuguese, of course. But they are surrounded by countries that speak Spanish and they have to trade with them and do other transactions. Why don't they speak Spanish? You'd think that they would have gradually learned it over time... The reason that they do not conform, is it because they are the economic power in South America? Or some other reason? So basically, why didn't Brazil conform and gradually become a Spanish-speaking country like all of the countries surrounding it? I'd like a reason that doesn't say 'because they were colonized by Portugal'.