Hola Tony

We can use both the preterite and the imperfect to refer to past actions as you know, but when we want to express that action as something more descriptive, more as a background information without considering it complete and finished we use the imperfect; this is what we are doing for example in the sentence "Estaba en México". 

The second sentence could also be using the imperfect as a description of something in the past "Era una civilización..." but we chose in this instance the preterite "fue" to convey that this is an action seen as a completed action with a beginning and an end, so we have in mind the limitation of that period, when they existed, and that had an end. 

Here is a lesson that talks about using the imperfect or the preterite. Have a look.

In hope it helps.

Saludos

Inma