it's high time that 后面可接两种形式,+sb did或者sb should（这个should不能省）

It is (high)time (that)sb did/should do 是句型所要求的,无理由可讲；而It is a pity that...句型可以用陈述语气也可用虚拟语气,用should表示竟然,should可省略：
It is a pity that you didn't come.It is a pity that it comes at such a cost.
It is a pity that he （should ）give up this chance.真遗憾,他竟然放弃了这个机会.
It is a pity that such a thing (should) happen in your class.这种事竟然发生在你们班上,真是遗憾.

it's high time that 后面可接两种形式，+sb did或者sb should（这个should不能省），可为什么同样是这种句型的其他句子，should就能省略，比如it' a pity +that （should）+动词原型？,,,