Why public parameters generation ceremony was not open for anyone?

I love the idea of zcash. However, I am disappointed by the public parameters generation ceremony. I expected it to be open and to have more participants.
Why participation in the public parameters generation was not public? Why use only 6 people? If I had a chance to generate a part of the secret myself, destroy my part, and verify that it was actually used for the final result, then I can be sure that the system is legit in that aspect.
I understand there may be issues with people not performing their part, but it can be solved by asking people to make security deposits in Bitcoin, and returning them in case of success.
Were there any technical difficulties not allowing to perform an open MPC process with several thousands of people participating? Any other considerations?