For a simple one word answer, Pride.
What started out as people who shared the same relationship with God, through Christ, meeting together to share in their joy and peace was marred and eventually upended by a few men who felt that they knew better than the others and began to force their ideas onto the others in the church. Eventually, some other proudful individual would form his idea of the truth that was in contradiction with that first overbearing individual and would lead a portion of the church away and a split would occur. To protect against this, a few 'proud' individuals evolved into a governing body of sorts in order to keep overbearing individuals under control. Then 'religion' as we know it today, was formed.
Check out www.johndarby.org/difficulties/index.html to see what I believe is the common template for the degredation simplicity and the formation of all organized 'religions' as we know them today.
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