That reason is a corrupted U.S. Supreme Court.
In 1976 (Buckley v. Valeo) and 1978 (First National Bank of Boston v. Bellotti), the Supreme Court ruled that when corporations and billionaires purchase their very own politicians, it is constitutionally protected "free speech" rather than "bribery," which is how we defined it from the beginning of our republic until 1976. In 2010, the Supreme Court doubled down on its betrayal of American democracy with its Citizens United decision.
After those twin decisions in the 1970s, money from corporations and the morbidly rich began to flow into the coffers of the Republican Party, hoisting Ronald Reagan into the White House. (Democrats were then still largely funded by unions, and thus not so easily up for sale.)
The spigots of cash never turned off; the 2016 election was a $6.5 billion affair.
Read more here.