Because the s54 is an engine with the maximum amount of power being pushed out of it, while still running on 'street' octanes. Its reliability would probably come into question when being used in a more base model car.
I guess we will see just how reliable the s54 is, as some of the M3's are starting to breach the higher miles. I personally would not want to own a car with an s54 in it while not under warranty.. who knows what problems these cars will have in the post 100k range. Also its pretty hard for your basic at home mechanic to work on, considering it sports things such as drive by wire.
If the s54 proves to be as reliable as it is powerful, it truly is an engineering feat.