You never get a unanimous vote in a reality show. You never get a unanimous opinion in life. You have to convince people to make it happen when your benefit is at stake. Noone owes anything to anyone. It is a favor, not a given.
If I was in Dev's place, I would have tried to convince/negotiate with Shefali instead of cornering her and making her feel like a bad person. Did none of them actually watch previous seasons? It was her benefit at stake. Shefali had nothing to lose. The only thing Shefali lost is some credibility among the housemates but she can regain it in next week if she tries. On the other hand, Devoleena lost the queen title and an opportunity to be saved from nominations. I guess Devoleena started to consider herself Queen after having four people on her side. She underestimated the power of one vote and also misjudged the situation.