According to Politico, it’s because Harry Reid wants him there. Doesn’t want him giving any more stuff away to the Rethugs:
“None of the deals Biden has struck have aged well from the perspective of the Democratic Caucus,” said one Senate Democratic official aware of Reid’s face-to-face insistence that Biden be excluded.
“In December, McConnell went around Reid — and straight to Biden — to get a better arrangement for Republicans.
The GOP goal was to lock in Bush-era tax rates for as many Americans as possible, while Democrats hoped to raise tax rates on people earning more than $250,000 a year and delay the implementation of automatic “sequester” spending cuts that has been mandated by an earlier Biden-McConnell agreement.
But Biden, negotiating for the White House with McConnell, agreed instead to a compromise that permanently froze tax rates for families earning income up to $450,000 per year while halting sequestration’s severe spending cuts for only two months.
“You can see Sen. Reid’s thinking,” said one Democratic aide. “Vice President Biden negotiated it down to two months. That’s not negotiation. That’s capitulation. The entire caucus was furious.”