Why is much more profitable for a company to bring to market a new product than secon?

Question

While your statement is generally true, it is not always true.

The producer of a new product MAY enjoy some monopoly power for a time. Makers of an existing product are, by definition, in competition with someone else.

Now then your statement is not always true. What if the firm discovers a low-cost way of producing an existing product. The potential for profit could be quite large.