Because if you choose the Datetime property to Editable in Presentation tab, then you wont get the Read Only Format options where you can specify the datetime control format as explained over here. I dont see any option to change the date format that is being displayed in datetime control for the Editable type as per OOTB options.
If you choose the Auto option then you can find the "Display value using read-only formatting" checkbox through which you can enable the formatting to the Auto mode datetime control as explained in the attachment. Hope this will help in achieving your requirement, the focus needs to be away from the property to reflect the formatting on it which is expected behaviour.
Through Auto mode you can also edit the property value Renuka, but you need to enter the date format in dd/MM/yyyy then the date will be formatted to dd-MM-yyyy after focus moves out of field. Apart from this i dont see any option to perform your desired task on datetime control.
You can use text control and write your own validation to accept only dd-MM-yyyy format into the field and apply the validate rule in your property "Use Validate".