They were targeted because they were black Christians, and therefore low-hanging fruit as far as he was concerned. He wouldn't have targeted black Muslims or black Panthers. The "non-violent Christians" aspect of this is indeed pertinent.
Anyone know why he drove 100+ miles away from home to do this? There are probably 100 or more black churches closer.
He targeted this as you said, because they were “low hanging fruit”. Safe and easy to pick off.
He also said in his rant that because he was “alone” in his cause he was unable to go into the ghetto to fight.