为什么：there's less air pollution in the city than in the country.这里的than后面没有用that呢?the weather in wuhan is hotter than __that____in beijing.

There's less air pollution in the city than in the country. 这个句子只有一个中心词，即pollution.
比较的对象是“地域”与“地域”比，即in the city 与 in the country比。
当然也可写成 The air pollution in the city is less than that in the country.
此时，这个句子的比较对象就变成了“污染”与“污染”比，用that代替pollution

我不会用标准语法解释来理解，我首先是靠语感，我觉得可以这么理解，仅供参考。虽然整句是比较句式，但主句即than前的句子句式不同。the weather in wuhan is hotter than that in beijing.这里的that指代the weather,完整的是the weather in wuhan is hotter than the weather in beijing.而there's less air pollution in the city than in the country. 主句是there be 句型，如果加上that指代in the city 前的there's less air pollution显然是不合理的，可能是思维定式认为that 指代air pollution.

there's less air pollution in the city than in the country这个句子是拿两个城市的空气污染作比较,比较的对象只有一个,所以不用再用that了

如果在不影响理解和不产生歧义的情况下。可以省略