As stated many times, correlation does not equal causation. The attachment styles could be the result of different socio-economic qualities of the families. If the family is well off, for example, the mother and father can spend more time and money on the child, which will make him develop secure attitudes. However, since the parent is well off, the child will probably also be well off, but that was not because the child has a secure attitude, it was merely correlated with it.
I agree with this because the attachment style of a person cant be directly linked back to the amount of money or things a person has. There are plenty of examples of how people who grew up in poor situations or with a negative family background grow up to be successful members of society and have meaningful relationships. I think the combination of events, role models and personal beliefs are what really makes the most difference in how a person grows in their attachment styles.