Because when you deposit to Kraken, your balance is NOT determined by the address balance. Kraken, like most exchanges, give people a unique deposit address for their account, when you deposit to that address, they credit your "account balance" with the same amount of coins and then they move all the coins into a "holding pool" of coins.
You no longer have any control over those coins, and they just get mixed up with all the other coins that Kraken has. Think of it like going to the bank, you give them $100.00 in cash... they just put it in the cash drawer with all the other cash, and update your account balance with +$100.00... when you ask for $100.00 withdraw, they just take whatever $100.00 is in the cash drawer, it isn't necessarily the same $100.00 you gave them
As for why the balance isn't showing even though the transaction is confirmed... sometimes the exchanges are just a bit slow
