Because Suhani is the living, breathing result of Nachiket's extra-marital affair with Roopali.
And she is standing right in front of Ragini.
Roopali (presumably) is dead and gone. Otherwise, she would also be on the receiving end of Ragini's wrath.
And while Suhani was out of sight for 15 years, she was out of mind.
In fact, having Nachiket around these past few weeks, rekindled Ragini's old feelings for him. She was beginning to be drawn to him again, forgetting all the past bitterness for a few brief moments. She is still in love with him. She was blocking out the reasons for the divorce, and warming up to him.
Then Suhani hops back into the picture. And unknowingly brings back some bitter memories for Ragini.
Of course, Ragini will direct all her anger and hurt at Suhani. That's just human nature.
I wish there was no Suhani.
I can't deal with another child. Specially as the writers are doing such a poor job of writing for the other four.