RLSP, whose president and MP Upendra Kushwaha is a Union minister, said 'Kushwahas' and 'Bhumihars' felt left out as the party was not given any seat to fight elections to the 10 assembly segments.
BJP, which held six of the ten seats, could win only four of the nine constituencies it contested while Ram Vilas Paswan-led LJP lost the one seat it fought.
He said bypoll results were not a reflection on Modi's leadership as he was not a factor.
The top NDA leaders in state, he said, had shown "flip-flop" attitude to Modi's leadership when his name had come up for prime ministerial candidate.
In an apparent dig at BJP leader Sushil Modi, the party said, people doubted that opposition was being engineered by the then Chief Minister Nitish Kumar.
While Narendra Modi hails from backward castes which rallied these communities behind NDA in Lok Sabha polls, the alliance lack a backward caste leader of this stature at the state level, it said.