From what I know, they did it for political power and didn't want anyone to challenge them.
Midwives, herbalists and other knowledgeable people were killed as they were seen as a threat to their rise to power. It was about greed, wealth, ownership of land and control of the masses etc.
Those people who held knowledge were accused of Satan worshiping so they had a reason to be killed from those "higher up". If there was only ignorant people or those too afraid to question their authority, then getting power over people was easy, especially when every one else was too scared to rebel.
But, hey, that's what I know and there may be other reasons too.