Ross Brawn has said he could not work with Bernie as their approaches are very different.
Brawn says he likes to have a planned long term approach where as Bernie tends to react to what is happening now and let the future be what it is.
Brawn said that he never made it a condition that Bernie not be involved.
Perhaps Liberty has the view you cannot have the two opposing approaches and took the one that they thought was best.
http://classic.autosport.com/news/report.php/id/129089