Why is it hard for the rich to enter the kingdom of God?

Jesus wasn't saying that at all really because it contradicts everything the scriptures say and He knew them expertly, (read Psalm 37:25-27, for example). Jesus was pointing to a very specific 'rich young man' who somehow managed to avoid entering the kingdom while still amassing wealth. This is what was so unusual. Usually the 'wealthy' are the blessed of GOD who are active members of the kingdom and who bless the poor and deliver them. The poor have no hope without them.

This particular 'rich young man' gained all of his wealth through unrighteousness even though he made the claim right in front of Jesus to have obeyed the entire Torah from his earliest youth. Yet when Jesus told him that he lacked only one thing, to give away all of his possessions, the man mourned and went away distressed.

Why?

Jesus called his bluff. The righteous wealthy routinely give away all they have as part of business, since righteous wealth is earned ONLY by providing for the basic necessities of others. All other commerce is what Jesus called 'Mammon.' There is no way to be sure that 'Mammon' will return to you if you give it away. However, if you give your righteous wealth away to the poor, what will they immediately spend it on?

Basic necessities. (LOL). They will give it right back to you. Once you are 'righteous wealthy' there is no way to be poor ever again. 

So Jesus was saying that it is easier for a camel to squeeze through a needle's eye than it is for a man rich in unrighteous wealth alone to talk himself into giving away all of his possessions. 

Jesus did not condemn unrighteous wealth, however. Only the decision to serve it rather than GOD. He said we can only serve one master, GOD or Mammon. When GOD blesses us with this 'other' kind of wealth, Jesus commanded us to use it to 'make friends,' (Luke 16:9), by delivering people from oppression, (typically the kind caused by government taxation or criminal debts). As a result, He assures us that we will have 'eternal dwellings' among those friends. By which He meant a permanent safety net against poverty.

Between the two, the righteous man has no possible way to go back to being poor and can spend the rest of his life helping those who need help. That is exactly how GOD expects and intends for the righteous to live. 

The Lord told us to forsake all, including status, fame, wealth, material enjoyment, and so on—all of which can prevent us from following God. ... The rich man's giving up his life instead of money showed us that for him to enter the heavenly kingdom was indeed as difficult as a camel getting through the eye of a needle.

Because the temptation is always there to make money your God. To make it the guiding force in your life, the force that guides all your decisions.

It is a simple matter of priorities, "Seek ye first the kingdom of God."  Mt. 6:33.

Because the rich usually get rich by taking advantage of others.

Plus, the fact that heaven does not exist makes it tough to go there.

Because the bible is a collection of folk tales.

Because the more you have, the more you want. Greed, and if they offend the law they can just pay it off. Those who are born very wealthy often struggle with empathy and compassion, since most of the world is poorer than them, and they are not forced to care about it.

Because if they need something, they just buy it. They depend on God less than people with nothing. I figure that is what that means. 

When you're rich you depend on money and materialistic things to give you happiness and you forget all about what God's original purpose for you is to keep on living forever on a paradise Earth which is restored after the resurrection. when you're rich you completely lose sight of that goal and your goal is immediate satisfaction.

And you never prepare yourself to accept or to do God's will because you're busy doing your own will

The wealthy have a tendency to rely upon their money to see them through hard times. They feel no need for God, so they never get close to him. Their money is their god.

A favorite prayer of mine...

Proverbs 30:8-9 — Give me neither poverty nor riches!

Give me just enough to satisfy my needs. For if I grow rich, I may deny you and say, “Who is the LORD?”And if I am too poor, I may steal and thus curse God’s holy name.