1.Because God is JUST.
And if God is JUST, He cannot condone the elect's behaviour and hence they're unable to enter His dwelling.
In order to appease his own anger against the sin of the elect, His justice has to be satisfied.
His justice can only be satisfy by the death of His Son Jesus.
Imagine Satan taunting God saying.
See I told you King David deserve to go to hell because he has commit adultery.
How can you allow King David into your dwelling?
How can you say you are righteous(JUST) and yet still allow a sinner like king David into your dwelling?
A righteous God cannot be with a sinner.
But God said: I have redeem him through the death of my Son.
His sin has been justify before me through my son and I no longer am angry with him.
Hence he is now sanctify before me and am able to dwell in my present.
2.For instant if you wanna enter my home you have to obey my house rule.
You have to wash your feet before entering in or else I am not allowing you to.
3.Because God chose to own you.
John 15:16 New American Standard Bible (NASB)
16 (A)You did not choose Me but I chose you, and appointed you that you would go and (B)bear fruit, and that your fruit would remain, so that (C)whatever you ask of the Father in My name He may give to you.
For example I chose and decide to own a stray cat, this cat is no longer stray. It is mine.
Thus I have to take care of it no matter how tough the situation. Taking care of the cat has become my responsibility because I have chose it to be my cat.
Now if my cat soil my carpet, I have to clean it up for them because they can't clean themselves.
4.God won't have to sacrifice for you if He have no part with you. He loves you that's why he decide to redeem you through the death of His Son Jesus Christ.
Perhaps you can look at it at another way.
5.Jesus wash the disciple's feet aka dying for their sin to make them clean so that they can enter his house aka heaven.
The following are a better explaination extract from Paul Washer.
Reference from Paul Washer:
If God is JUST, he must judge you righteously. And a righteous judgement means your death in hell forever. We cannot say and be biblical that instead of being JUST, God is loving. God's Love must be JUST. God must satisfy justice that cry out against you because of your Sin. God Himself is JUST. And God is perfect and consistence in all His attribute. In order to pardon the wicked the justice of God must first be satisfy and the wrath of God appease. Someone must interpose. Someone must intervene. Someone must do something. And being there is only two party. One being God and the other man. We put no hope in man. God himself must intervene to satisfy his justice, appease His wrath and make it possible to express His love in salvation toward wicked man.