The Time Traveller is referring to the point in time when society had achieved such a carefree, easy life that there were no changes that needed to be made in their lives. As a result, humans stopped using their brains for creativity and intelligence. It is suicide because humans did it to themselves with their machines and technology, expressing the author's belief in the dangers of too much progress. Intelligent thought was no longer needed, so humans used their brains less and less. The decline of intellect led to the development of the Eloi. The Time Traveller compares the Eloi to cattle grazing in a field. Their only purpose is just to exist.
The Morlocks, on the other hand, had to retain their ingenuity and abstract thinking in order to survive. Their lack of food and their existence underground compelled them to continue using their problem-solving skills.