What was wrong with worshiping at the “high places”?

“The plurality of shrines inevitably reflected the local multiplicity of Canaanite Baal worship, implying a Yahweh of Dan and another Yahweh at Bethel.”

http://mosaicmagazine.com/picks/2014/10/what-were-the-bamot-and-why-did-they-make-the-biblical-prophets-angry/