Good question. The answer is that when people stop funding the temples, the religion passes into history as mythology. Mithra, Zeus, Baal, Isis and Hor...they've all gone where Jesus is heading--into grade school ancient mythology book reports.
George Carlin had this in his routine about religion:
"But He loves you. He loves you, and He needs money! He always needs money! He's all-powerful, all-perfect, all-knowing, and all-wise, somehow just can't handle money! Religion takes in billions of dollars, they pay no taxes, and they always need a little more. Now, you talk about a good bullshit story. Holy Shit!"