Pretty Simple.
When Jharkhand got seperated, all the mineral resources(Ranchi valley), industrial cities (Jamshedpur, Bokaro) or religious tourist places(Deoghar , Jasidih) went to Jharkhand and Bihar didn't get anything.
Simmilar happened with...
more... Andhra Pradesh as well when Telangana got seperated.
So both of these states deserve to have good allocation in budget.
More importantly, Andhra Pradesh one sided support central government and even Bihar provided 75% of seats to NDA(Despite having huge alliance RJD+INC+LEFT).