Some corporate operators don't think 121 pilots want to deal with everything you have to do as a corporate pilot -- in corporate flying, you're the ramp agent, the baggage handler, the cleaner, etc, etc... If you're coming from the 121 world and use to having a large team to help you out, it can be quite a transition and many 121 pilots find they don't like it. On the other hand, many pilots get into corporate flying and find it's the best flying they've ever done...