Elon Musk will keep Spacex Private until the Goal of Human Colonies on Mars are Achieved

Elon Musk has said that there will be no initial public offering (IPO) of SpaceX stock before humans have begun to settle Mars.
 
“No near term plans to IPO SpaceX,” Musk wrote in a short message posted to Twitter June 6. “Only possible in very long term when Mars Colonial Transporter is flying regularly.”
 
The Mars Colonial Transporter is a conceptual vehicle that Musk has discussed as part of his company’s stated long-term goal: sending human settlers to Mars.
 
Solar City and Tesla Motors are now publicly traded companies which provide Elon Musk with about $3 billion of his net worth.
 
Going public with Spacex would constrain his ability to achieve ambitious goals like settling Mars which would conflict with quarterly profit maximization.