Let me introduce you to Tammie:
SHE was the captain. So nobody alerted him, but her.
On the other side, she realized they had lost an engine, and she realized they had a decompression problem, she and her team took immediate actions that lead to the Incredible successful landing.
Perhaps you mean why she didn’t realized that the engine had blown out and it had catastrophically failed. And yes, it was possible because the plane is a big apparatus, and some parts are not visible from the cockpit view. Take a look:
So, although you receive all engine failures alerts and warnings, and you feel all the aerodynamic changes, you can’t know the extent of the damage without actually looking at it.
EDIT 1:
I didn’t know people would start arguing if a camera would have helped. The plain answer is NO, all the information the pilots needed to safely land the plane was in the cockpit. The extent of the damage, it’s from interest of the NTSB, the involved institutions (airline, manufacturer, etc), and insurance company, but while attending the emergency, that information is unusable. I’d bet, if the cameras would have been installed as proposed in some comments, the crew wouldn’t have used them, because it isn’t essential to aviate, the first duty of aviators, it may be distracting actually.