why film should be thin

When light travels in a film, there bound to be absorption taken place, and the light intensity would decrease. If the film is thick, light reflected at the bottom of the film would decrease in intensity substantially because the light needs to pass through twice the thickness of the film.

Hence, when the reflected light (which is of decreased intensity) interferes with light reflected at the top of the file, complete destructive interference would not occur, because of the difference in intensities (hence amplitudes) of the two light waves. Therefore, what you would observe is no longer an interference pattern, but only a patch of reflected white light similar to that observed on mirror reflection.