Mexico is an "emerging market", "developing country" or "newly industrialized country" because although it is the tenth largest economy in the world (USD$1,845 billion for 2013), it has an ongoing industrialization, where agriculture activities are still important but are rapidly being outpaced by industrial (specially manufacturing) activities. This means Mexico is not fully developed, as many regions within the country have a quality of life equivalent to those of Western Europe, whereas there are other regions with education, life expectancy and economic advancement equivalent to those countries found in Sub-Saharan Africa.
Rural decline and immigration into the cities are the main reason; this is because Mexico is in the middle of an industrialization process.
Few economic opportunities and increasing industrialization
Not until after World War II because up until that point Mexico focused on Agriculture and never followed the U.S. in industrialization which was clear when Wilson's army invades Mexico with barely any resistance after attacks from Poncho Villa. Real Mexican industrialization can be seen mostly now.
Since early industrialization in the 1940's
Few economic opportunities and increasing industrialization in the cities.
India or Mexico or China. They are all going through rapid industrialization.
As a close neighboring country to the US, they shared the benefits of US industrialization.
Rapid industrialization coupled with lower prices for agricultural products.
All of them are developing countries, with an ongoing, incomplete industrialization.
These would include oil, gas, iron, copper and coal -- these have helped Mexico become an emerging market, with an ongoing industrialization.
It is a 'mixed' culture in this regard. There are regions quite industrialized, such as Monterrey in northern Mexico. There are other places such as Chiapas, where industrialization hasn't taken hold yet.
Virginia Webb Langdon has written: 'Social and economic changes in rural Mexico as a result of industrialization'