Why is there injustice in this world?

This is because this is not the Universe of Satya Purush. It is futile to hope for justice in this universe of Niranjan.
Sahib Kabir has beautifully said—

Dhanya Niranjan Tera Durbar,  
Jahan Tanik Nehin Neyae Vichar.  

Oh Niranjan! Strange is thy Durbar. There is nothing like justice to be seen here.

Karam Kravat Aaphi, Kashat Dait Puni Jeev Ko  

He himself makes the Beings engage in all sorts of
karmas and then punishes them.
Sometimes he makes the Individuals perform noble
deeds while at other times he makes them do wrong
Karmas. Just as in any other country, we find different
laws, in the world of Niranjan there are laws formulated
by him as per his own choice. Niranjan himself is the king
of all that is wrong and mischievous. As such, these very
things are found everywhere in his Universe. It is by
cheating and deceptive behavior that Niranjan succeeded
in getting all the Souls from Param Purush. Again, he
succeeded in having Adhya Shakti through deceitful acts.
He turned the relation of Sister into that of a wife. Further,
through deceitful means alone he imprisoned the Souls in
bodily cages thus preventing them to go to Sat Lok.
Therefore in this universe, often we see bad people rising
on the basis of evil deeds whereas the honest persons are
seen laboring hard to earn their livelihood.
The worldly people are seen following the evil
natured Sages while the real Sages and Sants have to face
denunciation. In fact, a true Sant has to labor hard to make
the people understand the real and true form of worship.
So the Individuals are made to attain heaven or hell
as per their Karmas. They are made to pay for the Karmas
performed by them in their previous birth. It is up to
Niranjan to make the beggar a king or the king a beggar.

Therefore, when the very basis of this universe is
falsehood, it is useless to hope for justice here.