Because of Sin.

They came to the gates of Canaan,
But they never entered in!
They came to the very threshold,
But they perished in their sin.
On the morrow they would have entered,
But God had shut the gate;
They wept, they rashly ventured,
But alas! it was too late.
And so we are ever coming
To the place where two ways part;
One leads to the land of promise,
And one to a hardened heart.
ANON.