Most clinics transfer five day blasts for DE. I think becaue they want to make sure they have divided and are expanding. My clinic will rarely do a 3 day transfer. Only if the batch is so-so and the recipient is willing to take the expense and risk.
I also think it is because they want to make sure their stats stay up and with a five day blast that has been watched and is healthy, therer is more of a chance of a pregnancy.