the US had more allies, during colonization, cause no one really liked the Spanish, proven when the US stole a half million square miles from them in the 1840's, in my opinion had the Mexicans still held that region which contains California (who, if they seceded would be the 6th largest economy in the world) and the entire southwest would be a lot better off. Also Mexico had a dictatorship for awhile which never ends well.
so in otherwords
1.) Few allies ,late independence :. slower development
2.) The US pretty much pillaged all their stuff
3.) Santa Anna who went to too many wars, consistently combating the US and wasting the nations money
4.) Its too far south to be of any GREAT trade importance ill show you http://www.frontiernet.net/%7Ekf2b/kf2b/images_raw/KF2B_Azimuth_Map.gif
as you can see from the azimuthal view of the earth by worldly scales, the US North East is mere seconds away from the then super powerful Europe compared to Mexico. (remember during the colonial period that distance was an extra few months to cover)
5.) colonial powers within the zones, Mexico was colonized by only Spain the north was England AND FRANCE AND OTHER EURO NATIONS this means a larger influx of people (mexico actually paid people to get them to work, and the majority of the population were religious leaders to "tame" locals)and inevitably more money.
6.) Finally it may seem like a stretch but language barriers, English at the time was really widely spoken unlike Spanish so unfortunately they were isolated from the rest of N. America as Canada and the US both spoke english.
Today Mexico's main barrier is probably the WTO and NAFTA which artificially make other nations more competitive then their own crops making their agriculture not as effective as a true free market system.
sorry for the caps got emotional. i love hegemony topics.
and Mexico did have slaves, they used more Native American Slaves and remember the Caribbean was held by the Spanish (and french) which was the main hub for slave trade in the New World.
