We don't know. That's what the reality is. All those people who claim pH or timing or "ions" or whatever sways - they were just guessing. They never knew any of that to be true or not, it was just their guess, and unfortunately then people took those guesses as proven fact and ran with it. But new evidence shows that while swaying IS true (or we wouldn't get good results with it) that all those old guesses are probably not true after all.
The newest science shows that it is probably a combination of lowered fertility (by fertility I mean the ability of sperm to make it to the egg) for you both as a couple, your overall maternal condition, and your blood sugar. Possibly hormones for you and sperm quality for your husband. But those things are just guesses too!
I think it's best for us to do what seems to be working for most people most of the time and not get too hung up on whys and wherefores, because then we end up with people doing all this stuff to follow a theory and overlooking things that are actually working!! So we have people taking tons of herbs for "lowering testosterone" or whatever but then they can't get pregnant doing that and end up giving up swaying totally, dropping everything and having another boy. I think it is best to just do the things that work for people, let go of the stuff that isn't (even if it supposedly is supported by a theory) and get pregnant quickly!