The RJD supremo, who has been in jail for more than two months following his conviction in fodder scam cases, made the attack on Twitter, which is managed by people close to Mr Yadav.
He asked Mr Kumar why he did not put enough pressure on Prime Minister Narendra Modi to offer special status to Bihar.
Mr Yadav's comments came in the backdrop of Telugu Desam Party ministers resigning from the Union council of ministers over the demand for special status to Andhra Pradesh.
"Why is Nitish not asking his boss to accord special status to Bihar. PM Modi had promised special status to Bihar in March, 2014 at a rally in Muzaffarpur during Lok Sabha elections. Nitish must play that speech recording in front of the PM," a tweet on Mr Yadav's official Twitter handle said.
He accused Mr Kumar of having surrendered Bihar's interest for personal gains - an oblique reference to the chief minister's exit from the Grand Alliance and forming a new government in the state with the BJP.
He also asked Mr Kumar to disclose why and because of whom Bihar is not getting its due. "Nitish ought to keep his spine straight and let go of his fear and his self-interest."
The RJD chief also sought to know how much of the Rs 1.65 lakh crore announced by PM Modi as special package has been given to the state.
"Actually, the state has not got even a penny. Admit it, if you have the guts. How long will you keep hoodwinking the people," the RJD supremo added.