You're just too much of a flight risk - they see you as having too little at stake in India. I've had some friends have similar issues (http://news.ycombinator.com/item?id=851287). I myself returned to India after 8 years in the states partly because it was practically impossible to start my own business without a green card, aka 4 years of indentured servitude.
For future reference: Try to show assets you own (real estate, homes etc) that incentivize your return; Bring literature about your conference, maybe ads etc (they need to see that its purpose is not an immigration scam); if you're married or have kids, that helps a great deal. Good luck!
Thanks for your comment. I brought a lot of documents with me during visa interview but didn't they see a single document. It's true I am unmarried but getting married just to get a US visa is too much to ask for :)
I am from Belarus. When I tried to apply for business visa to visit company doing me an offer, i was also denied. I had bunch of documents with me with assests, etc, but officer just took a look at application and refused issue of visa. So probably, from countries like mine if you are young and do not have kids, it's quite hard to get visa.
For future reference: Try to show assets you own (real estate, homes etc) that incentivize your return; Bring literature about your conference, maybe ads etc (they need to see that its purpose is not an immigration scam); if you're married or have kids, that helps a great deal. Good luck!