I have thought a LOT about this and the only sane reason comes to my mind that they wanted to balance Sanyogita as a loyal daughter and a loyal lover.
I mean, if Sanyogita had just run off with Prithvi than we would have said that she's a bad daughter. Right? I think they wanted to balance it to show that she was first with her father as any good daughter would be but when she realises that the man she loves is correct, her loyalty leans towards him.
This way, we can't say that Sanyo was a bad daughter, because she broke up with PRC as he tried to kill her Dad. AND, we can't say that she's a bad lover/wife because she does defy her father publicly and elope with Prithvi.
Also, the twisted Sagars thought that it would be the best track for the leap. I am not denying this at all.