They are the losers of society that can't or won't do anything for themselves.
You don't see educated people crossing the border, just the people that are too lazy to make it in Mexico.
There are just as many oppotunities in Mexico as the US, but with lower pay. These people look at the US as the saving grace for them and they can make a ton of money without doing much. It is only after they get to the states that they get a huge dose of reality.
On your other point, why would you think a soverign nation would let the US dictate what should be forced down the throats of the citizenry?
Are you of the opinión that the majority of the Mexican population are going to cross the border?