The answer, in this case, is not theological, but logical. It has to do with the limits of logic, which is conditional reliant on non-conditional (axiomatic, self-evident, etc) in order to begin at all.
If the first cause was conditional, it would be dependent, and the argument's point is to explain the existence of the dependent set we observe - not add another one to it.
In other words, out of the thin air. Anyone can build up a logical argument if the first argument is by necessity axiomatic and unconditional.
You said first cause argument is logical. If it is logical then it follows that first cause created and exists just because, for no reason whatsoever. Any other reason suggested destroys the argument.