If you don't show up for the games you are assigned they will stop calling you. The games available may not me suitable for you level. There may be other reasons too. Call the assignor and ask them. A list of assignors is available at the following location.
Tournaments are a great way to gain experience.
If you are a new referee, you get the chance to work for some of the areas more experienced referees. The assignor will team you up with one or more referees who can watch you and help you with tough situations. You will not me 'left out to dry'. If you are an experienced referee, you will get the opporunity to work with newer referees. You can share with them the experience you have gained.