The reasons for the population explosion of the 19th and 20th centuries became clear shortly after society crumbled, sometime in the 21st century.
It was at about this time, sadly just too late for many, that a power source both clean and immeasurable was discovered. Such was the depth and beauty of this source that engines for harnessing its power could be built with the simplest tools by anyone half conversant in the new technology.
The ready availability and exceptionally low cost of this power source had an equalising affect on the population of the Earth at that time. All material goods, personal gain and one-upmanship became something of a misnomer, once any person could produce as much of any item as they desired. There suddenly seemed to be no point to it all.
And, by and by, this lead to the invention of the time machine. It behoved people to return just a short time in the long history of this planet, (you could only stay on this planet; travel to other solar systems or even other planets was clearly a complete waste of time, due to the ridiculous distances involved), to the period perceived as the ‘boom-time’ in the long story of the Earth – the industrial and technological revolutions of the 19th and 20th centuries.
It must have been a great time to be in, went the reasoning, as there were so many people around at the time…
As you may be able to tell by now, what actually happened is that almost half the people on the planet at that time were in fact not from that time, but from a short time in the future.
The situation was such that you had people leveraging their knowledge and savvy to become the oligarchs of that famed period in hostory, whilst certain ‘thrill-seekers’ spent time in the most overcrowded and inhospitable areas of the world – living life on the very edge of survival.
It got to such a point that these people, somewhat foolishly, did not keep track of the date. Hence, society duly crumbled, due to obvious reasons, and the majority died with the rest of humanity.
Those that were left after that horrendous time had a different perspective on it all, and promptly invented the new power source – and so it went on.