Why has the success of egg freezing increased?

Freezing any gamete or embryo has several challenges the main being preventing the water within the gamete cell from crystallizing. If that were to happen, the cell membrane and other structure gets irreversibly damaged. Various sugars and proteins and such combinations have been used historically to prevent this from happening. Traditional wisdom means that the process had to be done slowly. In the late nineties radically different way of freezing gametes was tried which was called rapid freezing or vitrification. Over the last ten years that technique has further improved to the point where in the old method of slow freezing is practically less used and may soon become absolute. More than anything else it is a development in the art of vitrificaiton which has revolutionized the survival of oocytes after freezing. In the past the chance of oocyte surviving freezing and subsequent thying was less than 50%. Today the chance of this survival is above 90%. The process hardens the lining of the oocyte and thus necessity its ICSI than doing simple IVF.