Using "That" would be more natural. If someone asked you why you use tours, then it would be natural to say "It is because", because you are referring to the reason for what you said, and not specifically what you said before. For example: You: Whenever we go abroad, we use tours. Them: Why's that? You: It's because if we get into trouble... etc etc. But like Robert said, it's not incorrect to have "we use tours. That is because", but it's much more natural to say "we use tours because..."