这里为什么不用that?

which可以指代前面提到的句子,也可以是个别词

that不能指代句子

我觉得```
指代前面一句用WHICH

home后面加个逗号会清楚点，后面跟的是个非限定性定语从句，不可用that引导。非限定行定语从句，从句修饰主句，主句表示事物从句用which引导，主句表示人用who或者whom引导

这是非限定性定语从句,that引导的是一句限定性定语从句句子,而此处只是做home的非限定性定语罢了!

这个句子其实是
It was common in the tratitional family structure that children are likely to have less supervision at home.

在这里,it做形式主语, 代指children are likely to have less supervision at home整句话.这个句子里的that是个连词.

在你给出的句子里, 所选的词应该即关系代词, which能代指前面整个句子,即children are likely to have less supervision at home. 而that则不能代替整个句子.