Why 3D movies are made in less number.  

Well for one, the technology for 3D has only very recently gotten very good. Another it doesn't translate well into home viewing. But a lot of people don't want to do 3D because they feel it is simply a gimmick to try to rope people into viewing films which a lot of 3D movies are. And finally, some story lines just don't call for the film to be in 3D. It would be odd.

3D movies, while cool in movies like Avatar and a few others can be very childish and distracting in other movies. The reason a movie like Iron Man 2 wasnt in 3D is because it would have reduced the effectiveness of the movie. Another reason there arent more is because it costs more to produce the movie, and it costs more for viewers to watch it, so it could reduce the amount of money they receive from the movie and increase the cost of producing it. This is not something a movie producer wants to have happen for obvious reasons.