He's giving as much cover as he can to Sanders without explicitly denying it.
Biden can't manage a campaign. That's basically all there is to it. Fundraising, ads, field organization, etc. He's just been coasting the whole time. If he had Hillary's skills in that area the race would look very different.
Trump's been taking undeserved credit for this for years now. I'm pretty confident that the reason this doesn't come up is because of the nature of the reforms- they allowed vets to go through private providers instead of directly through the VA, which is very difficult to tout at the same time you're pushing M4A even though it was a big positive change.
