A combination of talent, strategy, and being round at the right time.
Sushant is debuting at a time when TV actors and people not related to BW are also being taken seriously, offbeat characters, interesting scripts are being hailed in comparison to glitzy glamorous candyfloss stuff. Vicky Donor struck gold at the box office even with a debut actor. That gave people more confidence to experiment with TV actors. Rajeev K. didn't have this luck, if he had debuted with Aamir today, things might have been different for him also.
Also Sushant groomed himself well, didn't just get into films after leaving his show. Worked on diction, made sure he got noticed well on TV, got noticed as a fab dancer, left his show amicably, no controversies. Even though he was a star already, he started from ground level- gave auditions like hundreds of hopefuls- was selected for KPC amongst 150 people. PK happened after Raju noticed him in the trailer and asked him to do a small role in it- again he accepted it, his humility did him good, PK is a big movie, people started taking him more seriously.
And he came around at the right time- when YRF is looking to launch newcomers. He has an edge cos he has experience and popularity already, plus two films in his kitty, and YRF was already looking forward to make a dance film.
And finally the biggest thing- he didn't just pick any role he got. You are right, Tv actors usually make debuts with small films. Sushant could have done that too- he was offered nine films before KPC from okay okay banners, like Hate Story. But he choose to resist the temptation, work on himself first, let the audience get over his image as Manav and choose a project which would do him good, even if it meant making extra efforts for it. (losing weight, training in cricket, gujarati workshops etc.) Sometimes letting go of wrong projects is very important, especially when debut film is concerned.
This is what I think.
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