Here the typesetter for the 1841 British edition accidentally misread this as his. The resulting text, “because his people repented not”, makes no sense here. The 1849 LDS edition restored the correct this. For other cases where the 1841 typesetter mixed up this and his, see under Helaman 8:22.
Summary: Maintain in Mormon 3:15 the determiner this in “because this people repented not”, the reading of the earliest textual sources.