Answer:
because the mutagen induced tumor causing mutations in somatic cells, not in gamete producing cells in the germline.
Explanation:
The gene can be transferred either horizontally or by vertically.
The above scenario is related to horizontally transferred of mutagen by tumor cells to the non-exposed mutagen mice. By transfer of mutagen cells, tumor develop the mice. When mutagen-induced tumor mice are mated with unexposed mice, there will be no development of tumor into the progeny.
It clearly shows that mutation occurs in the somatic cells only due to mutagen and it does not affect the gamete producing cells in the germline.