It will be interesting to anlayze the vote share and areas in which BJP did good, but without going to that data analysis (because in Indian journalism, mahaul is more important than data), my initial thoughts would be these (in that order):
1. Nitish Kumar was, to an extent, able to execute the ‘Bihari vs Bahari’ agenda. This was straight out of Narendra Modi’s book who could earlier successfully turn attacks onto him as attack on Gujarat. Perhaps Nitish Kumar is much like Narendra Modi, and likes repel each other!
2. Due the above, lack of a local CM candidate or a local leader promoted aggressively hurt BJP. I don’t know what are the personal ambitions of likes of Hukumdev Narayan Yadav, but he would have been just perfect as star campaigner.
3. RSS chief’s statement on Reservations hurt BJP’s prospects. Lalu was able to make it a big issue just as BJP could make beef a big issue in the last phase. Both benefited (some TV reports suggest BJP doing good in the last phase) from their respective spins on statements. BJP needed the RSS chief’s statement to be spun as EBC+Mahadalits vs OBCs, but it couldn’t do it, because it was also targeting OBCs (but wonder why no local OBC leader in that case).
4. BJP’s inability, or incompetence, to learn from Delhi results. It comforted itself by believing into conspiracy theories that it lost the poll on purpose (I too maintain that Delhi was a good election to lose, but it was not lost on purpose, it was lost because BJP was outwitted on every count). If they don’t learn from this defeat, 2019 is gone. I had written about Delhi elections here http://www.rahulroushan.com/2015/02/bjp-helps-kejriwal-become-the-cm-of-delhi-pm-next/
5. Caste continues to reign supreme in Bihar, rather in the Hindu society. While a BJP supporter online might be paranoid about “libtards” and Islamists and would be desperate to go beyond caste, the average Hindu, especially in areas like Bihar, can’t understand what the fuss is about (I had referred to this under “ideology” section in my write up on Delhi). BJP tries to change that through polarization and mostly it works, but it can work only in certain areas like the last phase. The ideological agenda has to go beyond the silly polarization.