I've got a really quick and easy answer to this question.
There are so many Christian denominations because;
'For the time is coming when people will not endure sound teaching, but having itching ears they will accumulate for themselves teachers to suit their own passions, and will turn away from listening to the truth and wander off into myths.' (ESV)
2 Timothy 4:3-4
The truth of God makes people uncomfortable. When they decide God's truth isn't enough for them, or when one part is particularly difficult for them, they turn to whoever is willing to tell them what they want to hear.
We need to quit with all division and unite together as one body under Christ Jesus our Lord!