Frankly the idea of a NATO country not part of the visa waiver is absurd. If the citizens of a country are worthy enough for the USA to be ready to send its youth in their defense, they probably are ok to enter USA without visa.
The US has a massive problem with illegal immigration from Mexico and Romania is even poorer than Mexico. It wouldn't make any sense at all to have visa waivers for citizens from countries that poor.
I think that the US is suffering an illegal immigration problem stemming from Central American countries and not Mexico. There's a downward trend observed from immigration to Mexico to USA in aggregate numbers (legal plus illegal) and for legal migrants, the numbers are heading in the other direction with more people from Mexico going back home than coming to the US.[0]
Please note too that Mexico is going through an economic boom esp. in manufacturing that's fueling this trend.
