The general argument for forcing someone to turn over the key to a door once compelled by the court is that it is commonly known there are other reasonable means by which to access the content the door protects that do not require the key:
1) Locksmith 2) Break the door down 3) Use a different door if one exist
These are reasonable methods as neither of them take a considerable amount of time, in many cases a reasonably consistent amount of time between unique cases, and are methods that are readily available in all markets and jurisdictions. This makes the case that the contents behind the door can be accessed through various methods, some of which do not require my involvement, therefor my not turning over the key is only nominally delaying the inevitable.
In the case of an encryption key that protects data, which once protected is only in the possession of the individual who also posses the key, this is very different IMHO.
It is very different for this simple reason:
If proper procedures have been followed, and certain encryption methods have been utilized, there is no reasonably consistent method for acquiring the original data other than information I posses in my mind.
I say reasonably consistent because if you had limitless resources and limitless time you could try every possible combination and at some point gain access. That is, however, unless the mechanism that relays the encryption key is not designed in a way as to destroy the data after N failed attempts.
In either case, no court can be reasonably certain of the time it would take to access the original data or if they would ever gain access at all. This directly conflicts with my right to a speedy trial and certainly conflicts with my right not to incriminate myself as I'm the only one who can provide access within a reasonable amount of time.