If the buzz is true, Union Home Minister Amit Shah came up with a lucrative offer for Thalaivar. If he joins BJP, Rajinikanth willn't only be made the President of Party's Tamil Nadu Unit but also would be declared as CM Candidate.
Superstar seems to be seriously considering the offer given by BJP to play the safe game. If he joins BJP, Rajini needn't have to worry about the money required to run a political party or election expenditure. Even the age factor (68 years) could be making him re-consider his decision to float a political party.
PM Narendra Modi and Amit Shah wishes to ensure BJP holds power in all the Indian States. After conquering majority of the North India, They are aiming to rule South India as well. In Tamil Nadu, Rajinikanth alone is capable of bringing BJP to power.