I was disappointed with the film.I had expected it to be much better.这句中为什么用it不用that,this或one我希望就对于这句而言

我看来
如果在不同的语境
that 也是可以用

固定用法
哈哈哈

it 代替的是the film,可以查找一下it的用法就可以知道了.

vt.
1. 预计...可能发生(或来到);预料;预期[+(that)][+to-v][O2]
He expected to finish the work by March.
他预期三月份完成这项工作。
We expected that you would succeed.
我们预计你会成功的。
2. 期待;等待;盼望
That's just what we expected.
那正是我们所期望的。
I'll expect you for supper at six o'clock.
我六点等你吃晚饭。
3. 认为理应得到;指望;要求[(+from)][O2]
Don't expect too much of him.
别对他期望过高。
I expect you to be punctual.
我要求你准时。
The professor expected respect from his students.
教授认为学生理应尊敬他。
4. 想;认为[W][Y][+(that)]
I expect you are right.
我想你是对的。
vi.
1. 期待;预期
2. (用进行时)怀孕,怀胎
Mary is expecting.
玛丽怀孕了。

因为it指代film 一般情况下我们用it指代前面提到过的东西.而不是that this one