Why did this transaction take so long to be put in a block?

I initiated a 7 BTC transfer at 6:13 PM Eastern Time from Coinbase to an external address in an over the counter transaction today. It was put in a block 4.5 hours later (10:42 Eastern Time)

Not conducive to doing business at all :(

I checked /tried these things:

• I saw many blocks had been mined since my transaction was initiated at Coinbase.
• I saw there was a fee on the transaction
• I tweeted an inquiry to Coinbase
• I saw Coinbase was connected to 118 peers
• I saw other Coinbase transactions posted after mine confirmed within 20 minutes.
• I tried to manually broadcast the raw transaction on blockchain
• I read Coinbase blog post that they had slow confirms in February.

Why did this transaction take so long to be put in a block?

I initiated a 7 BTC transfer at 6:13 PM Eastern Time from Coinbase to an external address in an over the counter transaction today. It was put in a block 4.5 hours later (10:42 Eastern Time)

Not conducive to doing business at all :(

I checked /tried these things:

• I saw many blocks had been mined since my transaction was initiated at Coinbase.
• I saw there was a fee on the transaction
• I tweeted an inquiry to Coinbase
• I saw Coinbase was connected to 118 peers
• I saw other Coinbase transactions posted after mine confirmed within 20 minutes.
• I tried to manually broadcast the raw transaction on blockchain
• I read Coinbase blog post that they had slow confirms in February.