Only because the state wanted the feds to pay for most of it. The feds paid 20 percent of the costs of building it, the state paid 80 percent, and it was agreed that the state was responsible for 80 percent of the maintenance. The state reneged on maintenance. Damage to the main spillway was observable in 2013 when a huge hole opened up, and the state elected not to do much of anything to repair it. They've been begging the feds for money ever since. This is Gov. Jerry Brown's problem, and he elected to do nothing... until the severe winter storms blew open the hole to a football stadium size recently, and again his solution is to beg the feds for money. Never mind he is spending the state's money on an unnecessary high-speed train, and begging the feds to pay for it. This is the same Jerry Brown who said the state can go it alone and build its own satellites and other infrastructure if the feds withhold money due to state sanctuary policies. F' Gov Brown.
The reason the emergency spillway is in use is because the main spillway has huge hole in it and cannot be opened up to full flow.
The issue is Jerry Browns refusal to properly repair the main spillway damage that became obvious in 2013
The Oroville dam is only slightly smaller than the Mosul Dam in Iraq and the possible failure of the Mosul Dam has been pitched as disaster worse than a nuclear war that could kill up to 1.75 million people.