Security reasons. As you drop a file, not the privKey, there would be a file transfer via rpc. Even though it is a very low risk, there is a risk.
Furthermore, the import function of geth (e.g. geth account import ./key.prv) works with private Keys, but not with json. mist wants you to drag and drop json files.
Edit: More precise:
The "pure" geth has no import function for json wallet files. It can only import private keys (see https://github.com/ethereum/go-ethereum/wiki/Managing-your-accounts#import-private-key).
The Mist "Import Accounts" functionality accepts wallet files in json format. But they can not be passed via Remote Procedure Calls (RPC) to geth, because geth has not Procedure to process them (it can only import private keys, not json wallet files).
You are doing rpc (mist --rpc), thus it cannot work and is greyed out.
If you are starting Mist without a RPC connection to another geth instance - thus just start Mist without any command line stuff - , then Mist and the node run in one context. That means, the Mist GUI can pass the json file into the key folder of the node (e.g. ~/.ethereum/keystore).
But RPC does not allow that (e.g. security reasons), because with RPC, Mist and geth do not run in one context.
And there is obviously no Mist menu entry for "importing private keys". Many (end user focused) Ethereum browsers don't have a "import private key" functionality, because this would bear security risk.