This is happening because the RadDateTimePicker is not designed to show so many data items. By default there is no ScrollViewer implemented in RadClock as we have decided that it won't be very usable to have so many items and to search for the correct one to select it. However if your scenario demand such behavior you can edit the control template and place a ScrollViewer around the ItemsContainer. That will allow you to render the items correctly.
the Telerik team