Hi old smokie,
In general, there are two reasons for assessment appointments being cancelled - either the appointment is no longer convenient for the assessors and another will be given or a face2face is no longer considered necessary now that the claim form etc has been scrutinised.
I'm afraid you will only know which of these applies to you by phoning, as the letter suggests.
If you are yet to undergo a face2face, you might find this useful
www.benefitsandwork.co.uk/forum?view=topic&catid=10&id=107189 You might like to bookmark/favourite this on your web browser now so that you can find it easily in future. This will allow you to return with further questions or comments about your ESA without starting a new topic each time