It was obvious for a long time that this was going to be an overwhelming defeat, I don't think it came as a surprise to too many people.
I think the biggest problem was the complete lack of any clarity on how the Voice would work. The government couldn't/wouldn't give any details on how it would operate, how many people would be involved, how they would be chosen, what it would cost and so on. We were being asked to change our Constitution but not being told what that change would really mean. All these details were going to be decided after the referendum was won.
This is what would have been added to the Constitution and it's the last part that was a problem for a lot of people. We needed to know all this first to be able to make an informed decision.
Chapter IX Recognition of Aboriginal and Torres Strait Islander Peoples
129 Aboriginal and Torres Strait Islander Voice
In recognition of Aboriginal and Torres Strait Islander peoples as the First Peoples of Australia:
there shall be a body, to be called the Aboriginal and Torres Strait Islander Voice;
the Aboriginal and Torres Strait Islander Voice may make representations to the Parliament and the Executive Government of the Commonwealth on matters relating to Aboriginal and Torres Strait Islander peoples;
the Parliament shall, subject to this Constitution, have power to make laws with respect to matters relating to the Aboriginal and Torres Strait Islander Voice, including its composition, functions, powers and procedures.
Also every state government and the Federal government have a Department of Indigenous Affairs and a Minister for Indigenous Affairs (or similar). If all these people were doing their jobs properly and actually working for, and listening to the people they're supposed to be there for, then none of this would be necessary. That's where the change needs to happen.