Leadership is all about change and people resist change. This article gives ten reasons: http://blogs.hbr.org/kanter/2012/09/ten-reasons-people-resist-chang.html
People resist change simply because change brings uncertainty. You are taking off for holiday. Do you not get any jitters?
So, don’t dig for reasons, others like the above article have done it for you. What are you going to do about it?
If you are passionate about the change, if you have much bigger picture of change, if you have worked out the process, if you are ready to go through the pain of some losses for yourself you just need to communicate all this earnestly and you have to stick it out. You will have people who will be willing to stick out too. With you. They are your people. The rest can take a walk.
If you have led well you would have around you enough number of people who are willing to change with you.
That’s it. Don’t fret about the reasons.