Because political leftism is a religion.
"With some notable exceptions, many of the communists I knew seemed to be essentially decent and intelligent people. But how was it that decent people like Sam and Lavender Aaronovitch – or my parents – could stick with the Party when they all knew, at some level, about the inhumanities for which the communist movement was responsible? And how was it that they stuck with it when it was becoming ever more obvious that the whole determined communist experiment was failing?"
"The answer, as Aaronovitch movingly argues, is that these people were human and flawed. They believed in the ideals. They believed that Marxism was true. They had faith for a lifetime."