As mentioned, some people absolutely do not want to be on camera. They get too nervous and cannot speak clearly or stutter. Other people are easily embarrassed and their face gets red. Or...once the cameras are rolling they get the shakes and cannot perform the sleights correctly. Trust me. I've filmed a few guys and it is more of a tedious process than one thinks. Rarely do you get an effect filmed correctly on the first take. The lighting could be off, the audio, mistakes by the magician, etc. It may take upwards of 8 hours to film a DVD.
Others know that they are a rookie or kind of a newbie in the magic field and if they let another "well known" guy who has established himself / herself that potentially more copies will be sold thus making them more money too.