I really am going to bed, but since semck has rejoined, I'd like to add a little more rigor to my argument.
Moving past the obvious atrocity, here is why I believe it was able to happen:
1) Average doctors (an all healthcare professionals) have been systematically removed from abortion clinics.
1a) Through violence and fear (either perceived or real) an average doctor who many normally enter this field now longer has enough motivation to risk his life (or feel he may be).
1b) Thus who remain in the field, do so for a more powerful reason.
1bi) One would hope, and I assume it to be true, that most people remain in the field because they have a compulsion to help women with unwanted pregnancies and they truly believe they are doing the right thing and are the only ones who can help
1bii) I would expect, however, that there are some who remain in the field for wickeder reasons. Either some odd type of war profiteering or, in this case, some criminally insane desire to kill infants.
The problem here is two-fold. First, there are simply fewer people to chose from then there should be, so it's simply more likely that a sociopath may sneak through. Secondly, the good doctors may be so busy trying to keep the clinics running, they may not notice or, perhaps, involuntarily turn a blind eye towards the darker aspects of the clinic.
2) Constant attempts to shut down clinics prevent proper legislation.
2a) There are many states (North Dakota is the most recent) that attempt to pass laws that are aimed at shutting down clinics.
2ai) This takes time away from laws that could actually regulate clinics
Not only do these laws perpetuate number 1, they create this hostile environment that causes clinics to not self-regulate as well as they should, fearing an us vs them mentality.