Bitcoin Transactions are not instant.
Bitcoin transactions have to be confirmed by the Bitcoin network.
When a transaction is made it gets sent into a transaction pool from where it is bundled up into blocks that Bitcoin miners confirm through mining.
Blocks get mined on average every ten minutes and only a finite number of transactions can fit into each block.
Sometimes blocks can take much longer or much less time to mine. On rare occasions there can be several hours between successfully mined blocks. This means that the pool of unconfirmed transactions grows larger than what can fit into a single block, making it take a long time for transactions to become confirmed.
Once the transaction has been included in a block and has been mined, it has been confirmed once. When the confirmation count of the transaction reaches a certain threshold, usually 3-6 confirmations, the transaction appears in the receiving wallets.
You can read more about confirmations on the Bitcoin wikipedia and you can see the current number of unconfirmed transactions on the Bitcoin network here.
To learn more about BTC blocks and confirmations; Click Here.