As others have alluded to, the closer you get to 100% the harder it is to squeeze out those last few points. As wierd says peak usage periods and poor production periods are a definite factor in that, and that the current storage technology we have would be very expensive (and potentially have high land usage) to cover the scale needed.
And it'd be really darned expensive - either you need to build enough capacity to be able to meet peak demand in poor weather (meaning you get up to a ridiculous overbuild for most other situations), and/or you spend a lot of money on storage.
There's the other, not so minor, problem of the way the energy sector works in America. It's a hodge-podge of national, regional, and local administrative entities interacting with a mix of monopolies, semi-nationalized entities, regulated markets, and other fun stuff (sometimes !!FUN!! stuff like Enron or PG&E). The ways those various segments are regulated is often pretty corrupt and not always incentivizing smart outcomes.
That's one reason why (as I understand it) Sanders would just nationalize the whole darn grid. Would be a probably-impossible (and possibly unconstitutional? haven't done the research on that, though) political lift, an administrative nightmare to transition, and probably a nightmare to manage. If one is gunning to 100% renewable on a short time frame, though, probably the most feasible way...
Though (again) as had been said, the benefits of squeezing out that last few percent probably aren't worth it. Better spending those resources more efficiently reducing GHGs from other sectors - these days the electric sector is only something like a third of US GHG emissions (transportation recently moved to #1, and I think buildings is #3).
That does have advantages though. If you tell twenty people you want to see 95% of them comply with X, you will get 0% compliance and twenty arguments why the other nineteen people should comply with X. If instead you demand 100% compliance, you will get 0% compliance and twenty arguments why they personally believe they shouldn't have to comply. It's easier to deal with the latter case piecemeal without being accused of playing favorites.
Plus which, politics is the only arena in which you want to ask the impossible in the hopes of getting half of it, because then everyone gets their own little carve-out to take back home to their constituents and brag about.
One of the risks here is if members of the party aren't in a position to demand 100% while still being able to win reelection (or at least feeling that way). In congress plenty of Ds (to say nothing of the Rs) don't feel like they can support it, which would make things rather awkward for interacting with their President. And unlike the situation with Trump & Rs, the Ds would actually have to pass laws instead of just looking the other way.