Bike is IMO a good supplement to other cardio, because it's less rigorous in a seated position that is not supporting most of the bodyweight while exercising. Recumbent's even less rigorous than a regular bike.
To make biking a primary source of cardio the overall level of intensity has to be increased. Spinning/HIIT are the most common ways to do it, either in a class, by following a Spinning class video tape, by timing intervals, etc. Watching others do it can help with motivation.