They had not been exposed to most diseases common to urban population.

我通过转换你就知道划线部分的作用了：

They had not been exposed to most diseases which were common to urban population.

形容词短语，作后置定语是常态。单个的形容词可以放在名词之前，词组就无法前置了，这就是后置的原因。再举数例：

1. an actor suitable for the part 适合扮演这个角色的演员。

2. a manager responsible for the work 负责这项工作的经理

3. a basket full of vegetables 一只装满蔬菜的篮子

4. people present at the meeting 出席会议的人