Differences between Obama and Sanders:
-Obama actually had significant establishment support. Sure, Clinton had more endorsements, but Obama (and Edwards) still had a solid base of support, and weren't getting blown out in the hilarious way Sanders and O'Malley are.
-Obama had a unique ability to appeal to African Americans. At first, AA's supported Clinton, but once Obama proved himself the historic nature of his candidacy became very powerful. There's no chance of Sanders matching Obama's appeal with African Americans.
-Obama was (and still is) a much more eloquent speaker than Sanders. Sure, Sanders' message is powerful to many people, but the delivery is only average, and honestly fairly repetitive. In general, Obama is much more charismatic than Sanders, also younger and 'cooler'.
-Finally,
its a myth that Clinton was seen as inevitable in 2008. She was never thought of as the defacto nominee by a significant number people.