The primary reason for low birth rate is availability of cheap and effective contraception (and cultural changes that led to most people being ok with using it). Change my mind.
The real questions here is why people given opportunity to not have children jumps at it.
My personal opinion is that children are not that great - too much responsibility, too little support from society, too many of them behaves like little psychopaths most of the time (this can be probably cured with strict discipline but stress free upbringing seems to be in vogue). And the things that children "give" in return are rather questionable (especially for people with low emotional needs like me)
Most people probably like the "idea" of children but the practical side of having them is mostly boring ungrateful work that takes greater part of you life away and there is inherent risk that it can never end (I do not understand how anyone would not take into considetation the consequnce of having special needs child)
Point taken, but that opportunity was not there until about 60 years ago. I don't think people unwillingness to have kids changed, they didn't have much real choice back then. Well, their willingness to have sex was stronger than willingness to not have kids ;)
BTW: 2.1 is still quite good. Here in Poland (a formerly pretty much catholic country), we are at birthrate = 1.3 now and still going down.