DDT ban is often cited, but it's an insufficient explanation: first of all there are still efficient insecticides available, and bedbugs infestations only reappeared many decades after DDT was banned.
The most important causes is the massive increase in travel/tourism (airbnb is often pointed, but regular hotels are where it started) then come the increasing cost of labor (Baumol's costs disease) + higher safety standards for both the inhabitants and the workers (this includes DDT but not only) + higher living standards (people have bigger apartments with more stuff in it), which makes house decontamination much more expensive that it used to be.
I think the most technically advanced and affluent societies were capable of utilizing the very powerful mid-century insecticides, not only DDT, to almost completely wipe out bedbugs from domestic environments for quite some time afterward.
Decades later with no comparable eradication strategy, and once world travel increased exponentially, they have been reintroduced from parts of the world where they have always thrived.
The population density has reduced a lot over the past 70 years, it has almost halved in some arrondissement (it's still among the highest density in the world though).
You can find quite a lot apartments that used to be two smaller ones that have been merged together, especially in the upper floors.
There is absolutely no way that bed bugs were actually on the brink of extinction. There were always going to be pockets of bedbugs that were avoiding pesticides that would come back whenever spraying stopped.
