Some reasons why it was so easy for libs to forgive Obama:

1. He's incredibly charismatic.
2. The closest thing to an organized left in 2008 was the progressives who coalesced around his campaign, who he immediately demobilized.
3. He was able to pass some signature legislation, including at least some attempt to unfuck healthcare and some attempt to address the economy.
4. The most prominent attacks against him were mixed with a heavy dose of overt racism.

Each of these things either gave libs some credible reason to excuse Obama or made it more difficult to effectively criticize him from the left. Biden doesn't have any of this, and would take office in a country facing worse material conditions than it did in 2008.