A federal policy change in the 1980s created the modern food desert

Unrestrained capitalism creates monopolies and those 3 presidents created an environment for monopolies to flourish.

Reagan started things off by deregulating businesses and rewarding big business for "efficiency". Which really just translated into big businesses buying out the competition and jacking up the prices.

Clinton continued a trend primarily by dismantling and privatizing the federal government. That lead to giant Quazi-government businesses that have little to no competition and ultimately bring the worst of all worlds. The rollout of the ACA marketplaces is a prime example of what that capture looks like, a sub sub sub sub sub contracted and ultimately outsourced project that was riddled with bugs costing millions to create when literally if the fed gov had a team of software devs employed (perhaps some of the devs from the DOD?) it would have cost far less and would have actually worked.

Bush then caused a crisis in funding by dishing out huge tax cuts. Those tax cuts (especially around capital gains) essentially encouraged the hording of wealth while simultaneously with Clinton's privatization of social programs caused a crisis in the federal budget. Couple that with 2 giant boondoggle wars where we effectively dumped trillions of dollars into defense companies to later abandon our purchases in the desert. Defense companies, I might add, that shouldn't exist as their only client is the US government. Why should we be funding 4 or 5 companies to build weapons for the US military? Why can't the US military build its own weapons? Why should we be purchasing wasted tanks with generals literally begging the US government to stop buying equipment? I would much rather that money go to something like UBI, food stamps, or SS/Medicare instead of being the most inefficient way to make jobs. Heck, we could dump that money into free public education and R&D to make sure we aren't falling behind technically without producing weapons that will sit unused.

I think that thinking of it this way shifts the blame unduly; it's not like Republicans as well as Democrats don't bear the blame for continuing harmful policies, just because those policies were instituted by Republicans. As to Democrats, the first Democratic administration that was elected after Reagan did so by running away from many traditional Democratic priorities, and both other Democratic administrations had to deal with a recalcitrant Congress for at least a substantial portion of their time in office.

Even if they hadn't, though, I think that the general fact is that it's much easier to create problems quickly than to fix them quickly, so that the idea that any administration, even a 2-term one with a hypothetical cooperative Congress, could undo such changes (as opposed to just starting to undo them) isn't plausible—or, if it could be done, then it would surely be as all-consuming of political capital as the ACA was, at the expense of pursuing any of that administration's other priorities—and being able to pursue a longer-term solution would require electoral patience with political promises that weren't accompanied by immediate delivery.