Because Secularism has now come to mean pandering to Muslim vote bank in India and a typical self-hating Hindus.
And rightly so in my opinion. The meaning of words changes with time and actions...the original Secularism - the Gandhian/Nehruvian Secularism is practiced no more by Congress and the Mandal era political parties. And consequently what Secularism has come to signify is different than it was decades back.
Well, we are not the only people who use the English language. English is used the world over, and fora like this on the internet are read by an international audience. It is a bit dangerous for us to use the "new Indian meaning" of the word, when it can be so easily misunderstood to mean that only communals can support Modi.
And it's not like the usage of the word has actually changed so widely in India either. The preamble of our constitution still uses the word, and I'm sure it is used in the good old meaning. Until such a linguistic shift actually occurs, we ought to be careful not to overuse the word in the new sense.