that is why is a clause

Reason predicate clause.

The main sentence is That is why. That is the subject, is is the linking verb, and the component of why in the sentence is the predicate.

The clause we all suopport his idea is a supplementary explanation (expressing reason) of the predicative why, that is, the reason predicative clause.

The subordinating conjunctions of reason predicative clauses include because (because), as (due to), since (since), now (that) (since), when (since), seeing (that) (due to, in view of) ), considering (that) (taking into account), given (that) (taking into account), for (for), etc.

Extended information:

The adverbial clause of reason expresses the reason. Common introductory words of the adverbial clause of reason:

1. The adverbial clause of reason introduced by because is generally placed After the main clause, the because clause should be separated by a comma when it is at the beginning of the sentence, but it does not need to be separated by a comma when it is at the end of the sentence. because expresses the direct cause, has the strongest tone, and is most suitable for answering questions led by why.

For example:

I do it because I like it.

I do it because I like it.

You can trust those products because the quality never varies.

You can trust those products because the quality never varies.

We went by bus because it was cheaper.

We went by bus because it was cheaper.

Note: The not in the "not... because" structure negates the entire clause introduced by because.

Baidu Encyclopedia—Adverbial Clause of Reason