"that is so because" is correct and usable in written English.
You can use it in a sentence when you want to introduce a reason or explanation for something. For example: "I think that flowers are beautiful, that is so because of their colorful and delicate petals."
That is so because, as we have held time and again, an official's crucial duties are better carried out late than never.
For my faith that is so because the central act of Christianity is God's self-sacrifice through Jesus' crucifixion.
That is so because any price in excess of the marginal cost helps amortize overhead and other sunk costs.
That is so because banks don't want to provide the kind of leverage that they did in the past.
That is so because even a coherent piece of legislation — and many are not — typically addresses a bundle of issue-related concerns, or the needs of a particular constituency.
That is so because the regime has widely publicized developments in his case and because, unlike other foreign policy priorities, al-Megrahi's situation involves an immutable timeline in the form of his approaching death from cancer.
In part, that was so because Italian cuisine had always been highly regional and did not have centralized standards.
"That was so because thermal coal was plentiful and cheaply available to the power stations from many sources," Fraser said.
That was so because the use of the land adjoining roads would be the generator of traffic.
We hope that's so, because the likely outcome of implementing the draft as written is that schools will admit only students who pose the least risk.
That's so because he covers it every time the temperature drops below 40 and puts a 100-watt light bulb under the makeshift tent.
