Because scripture interprets scripture. And the load is lightened elsewhere:
1Ti 6:17, Charge them that are rich in this world, that they be not highminded, nor trust in uncertain riches, but in the living God, who giveth us richly all things to enjoy;
1Ti 6:18, That they do good, that they be rich in good works, ready to distribute, willing to communicate;
1Ti 6:19, Laying up in store for themselves a good foundation against the time to come, that they may lay hold on eternal life.
Which is not to say that I am wealthy by any standard. Compared to most people in the United States I am dirt poor. But compared to people in third-world countries I am actually quite wealthy.
I have left everything behind to follow Him on not a few occasions; I was considered to have a mental illness because of it. And also, the people in my family prayed that I would return and because of that, I am back with them and the possessions that I had before.
If you cannot escape your possessions, I don't think that God requires you to forsake them again and again. There comes a point where you realize that what He really wanted was your heart; and that because He has that, it does not make a lick of difference to Him if you have possessions: because they don't possess you.
Also, in context of the passage that you speak of, Jesus said that it is hard for those who
trust in riches to enter in to the kingdom of God. So the context indicates that it is an issue of
trust.
And it is not that God only speaks it to the rich young ruler: for the context makes it clear that He is speaking to
anyone who is
trusting in their riches to help them when they are in a bind instead of the Lord.