Why Those Heroines Reject Heroes?

Long ago, we heard milky beauty Tamanna rejected an offer to do opposite to Prince Mahesh Babu's '1-Nenokkadine' movie. Currently, Kriti Sanon is doing opposite to Mahesh Babu in this movie. Some time ago, we heard chubby heroine Richa Gangopadhyay rejected to do the second heroine in 'Atharintiki Daredi'. This particular role has been done by Pranitha. Off  late, we are hearing many gossips about Heroines rejecting Natasimham Nandamuri Balakrishna's new movie. Also, there were rumours that Dusky beauty Samantha rejected an offer to do opposite to Ram Charan and Krishnavamsi combo's movie. 

Why are these top Heroines rejecting the offers of very big heroes of tollywood? Many film makers say it is quite a common practice that Heroines rejecting the movies of top heroes. Since it would be a tough task for them to adjust the dates, it is obvious for them rejecting top heroes' movies sometimes. However, some times gossip mongers spread these gossips, to defame either those Heroines or the heroes.