When the Force was a myserious, spiritual sorta thing, you'd expect people to connect to it randomly. But after midichlorians, something that can be quantified in a blood test? I don't know how they're meant to work (I'm sure it's explained further somewhere, but I haven't bothered with the SW EU since about middle school) but you figure they're either like really benign bacteria, or something produced by the body. Either way, you'd expect it to be heritable, with a certain amount of variation.
So after there's a genocide against the Jedi and the kids they had identified as future Jedi, you'd expect the pool to be a lot thinner.
But really, it's sci-fi, so they could resolve this a billion different ways. Some Jedi managed to lay low like Yoda and Obi-Wan! There are aliens/environments/machines which dampen or mask the Force! When the Jedi were killed the midichlorians dispersed to find new hosts! Luke was having gobs of unprotected sex off screen and has a few dozen bastard trainees already!