Pretty simple really, it is precisely a riff on HRC's overall strat. He's trying to court the moderates, who generally think the economy should be priority #1.
He has a finite number of things he can say, and what appeals to one group is less appealing to another. If you can think of a platform that might appeal to both newer leftists and the older boomer dems and the independents all simultaneously, then that would be better than what he's doing.
But otherwise he's going to feel like he needs to shore up his support wherever its weakest, that's just logical. So, he talks up his economy. He's not appealing to his base like Trump does, he's trying to recruit people that don't particularly like him. Trying to compromise, like a veteran statesman from a checks-and-balances government. Build coalitions. Reach across the aisle. All that crap.
My thoughts anyway.