Film producers approach channels that they feel will have the best target audience for their film, and together, they decide which show would work best with respect to the subject of the film. So, Ranbir Kapoor and Deepika Padukone will promote Yeh Jawaani Hai Deewani on Pyaar Ka Dard... because the character of Aditya (lead actor), is similar to that of Ranbir’s in the film. Back when the Bol Bachchan actors appeared on EHMMBH, they cheered up Maanvi (played by Nia Sharma) and urged her not to give up her fight against cancer.
A reason for this trend picking up may be lack of time for the stars to visit tier-3 and 4 towns to promote their films and give interviews to local newspapers there. A TV show is thus their best way of reaching out to audiences far and wide. In smaller towns, some TV actors are bigger stars than their Bollywood counterparts. So when the audience sees Bollywood actors interacting with their favourite TV stars, asking them to watch their film, the message goes across very effectively. It’s as good as a celebrity in an advertisement asking you to buy a particular soap or a brand of deodorant.