It seems as if porque would convey the same idea and is probably more often used in conversation. So is de tanto more formal, literary or is it used in the street?
This expression is not more formal than porque, I'd say it's simply more idiomatic and it's used with a different structure. Using porque to express a cause is very literal and a simple way to express a cause; on the other hand, when we use de tanto/de tan como, we are using a set expression which is also a bit more emphatic than the more simple porque.
I hope it clarified it.
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