He didn't give the reason,May be his daughter-in-law Aishwarya Rai Bachchan's father Krishnaraj Rai passed away in March after several days of illness.
Amitabh tweeted on Saturday night
More details on his blog
NO .. there is no celebration of any kind for the 75th .. and there is no possibility of my presence here in the city ..
they speculate .. they aver .. they get to know from their ‘sources’ , details which can be completely wrong .. at times one
wishes to be those sources - for we never ever know who or what they are … and the state of this state would be the largest
wonder .. but so is the business that that Omerta is beyond all .. the code of conduct, of silence, that they sign is
invaluable remote and unbreakable in the entire universe .. one that only justifies silence .. but one that has deep and
final penetration in the conduct of them that pay allegiance to it .