The reason can be summed up in one word: identity. We get attached to our identity and we don't want it to change. The problem is that the church is a living organism and all living things change. While the 'stuff' of what we're made of remains the same, our identity shifts with time.
Let my show you what I mean. When I was young I was part of a family where my identity was that of a younger brother and a son. As I grew older and my siblings left home then my identity changed. I was still a son that lived with my parents but my younger brotherliness faded into the background without sibling presence.
Later, I left home and met my wife. We married and had kids; I became a husband and father. I remained a son and a younger brother but those parts of my identity became less relevant as my ID as a hubby and dad increased.
Imagine what might happen if I failed to live into my identity as a husband and dad and still wanted to live only or primarily as son and younger brother. How would that affect my ability to be a good husband to my wife? What might my children think?
Churches have identities too. And those identities change. I've seen some churches celebrate their past without clinging to it. Like the son that grows up and becomes a husband. Those churches are able to embrace their history and let it go at the same time--in order to become something brand new. However, I've also seen churches hold onto their past and get stuck. Like a little brother who waits for his siblings to return and misses the opportunity to grow up and become something else. Those churches become rigid, oppressive, and eventually close their doors.
An essential part of embracing a new identity is being able to find ways to celebrate the past without holding to it. When that happens, the church is getting ready for change. And, as stated earlier, when that change comes get ready for conflict!