Dropbox depends on being able to deduplicate your data.
If they can't decrypt it, they can't deduplicate it.
If they couldn't deduplicate it, the costs would be higher.
Higher costs => higher prices
If they can't decrypt it, they can't deduplicate it.
If they couldn't deduplicate it, the costs would be higher.
Higher costs => higher prices
The encryption process doesn't result in one giant blob of data. It results in N blobs, where N is the number of files you store in Dropbox.
EDIT: I misread "deduplicate" as "duplicate". You're correct.
The second method would require identical files from different users to encrypt to the same form, meaning the encryption is not key dependent and thus can also be decrypted without your key.
There may be some other method I'm missing but I don't see how they could de-duplicate with key-dependent encryption when their users hold their own keys.