Because it's harder to corrupt a "pure" democracy. If you have a policy that is bad for the population at large, but good for your own interests, in a "pure" democracy then you will have to convince a majority of the voters to vote against their own interests.
It appears relatively easy to get a majority to vote against their best interest. Case in point: Trump getting elected. If all that's needed to win a vote is populism and ad money, then you can be damn sure that people will vote not just against unalienable rights of minorities but will also against sell out their best long-term interests to short-term ones or to "principles" that sound good because they've been constantly repeated on Fox News or CNN.
Even without demographics taken into account, a country that's not accountable to a base set of unchangeable rules is a scary thing.
I'm proposing that the concept of states is obsolete now and we need to move to what I'd call a "unified America" policy, where we get as close as possible to a majority rule system of democracy. The best way I can think of to achieve that is to abolish the Senate and the electoral college.
If you think our current system of governance is working well, then please say so. If you think our system isn't working well, then please let me know how you propose to fix it.