Actually one of many reasons.
Presby Peacemakers Attack Jewish Groups in the U.S.
My experience in the PC(USA) was in a community that was very concerned about social and political issues and unconcerned about theological ones. Had you walked into my (Presbyterian) Church when I was there, and said that Jesus the Christ is God and the Son of God, fully God and Fully Man, and asked if anyone there agreed with that, you would have gotten several reactions. The most prominent would have been outrage and loud argument. A very few would have understood the real concept of the Holy Trinity, and even fewer would know what the dual nature of Christ is even though both doctrines are officially at the core of the Presbyterian doctrine.
Rather than educating its members about the teachings of Christ and the early Apostles, the Presbyterian Church would rather play in politics and social issues. And with a very leftist view at that. Pro-abortion, pro-gay, pro-feminist, and pro-huge government while being anti-military were the essential characteristics of the Church when I left it.
The very fact that the IPMN even exists and succeeds as an anti-semitic organization says it all.
A gay clergy is coming to the PC(USA) soon.