Because they are diametrically Polar opposites in Political Philosophy.
Here's my favorite example of Modern American Fascism: Hemp/Cannabis. U.S. drug policy is inherently pro-special interest (Law enforcement and significant elements of the legal system, criminal cartels, pharmaceuticals, tobacco, and the liquor consortium), racist and fascist. BOTH parties not only heartily support it, but have strengthened it over the decades. Since 1937, Democrats have been just as complicit as Republicans. FDR signed the Marijuana Tax Act. Harry J. Anslinger, America's first drug czar (and arch-villain bureaucrat), was re-appointed by Presidents Harry Truman and John F. Kennedy. Anslinger retired in 1964 during President Lyndon Baines Johnson's tenure. But, the baby boom generation was transformed when the Beatles changed everything and their music achieved an entirely new level after experimenting with cannabis. LBJ ignored the hippy roar of the counter-cultural movement and it's embrace of cannabis. President Jimmy Carter mentioned ending cannabis prohibition, but did nothing. President Bill Clinton chickened out and didn't inhale after all. I'm sure the Obamanation will not feature the end of cannabis prohibition. Neither will the third Clinton term, if Hillary manages to steal the nomination and somehow win the general election.
Republican drug policies have been worse, but the Democrats steadfastly refuse to reverse Republican drug war escalation when given the opportunity. Republicans and Democrats BOTH expoit the War on Drugs as a means to an end. Just as the National Socialist German Workers Party exploited anti-semitic animosity as an excuse to persecute Jewry - Germany first, but eventually world-wide, which suited their goal of acquiring power. Power they used to scapegoat, imprison, enslave, and eliminate millions: Jews, political dissidents, homosexuals, gypsies, and the disabled like the mentally ill and mentally infirm. All this was fine with the average German living under the Third Reich because life was good long as it did not consider you a threat. As long as the average German went along they were not a threat.
Both parties use fascist tools to further their goals of acquiring and remaining in power. I can't think of many prominent Democrat political officials (elected or otherwise) who openly denounce cannabis prohibition. Fascism plagues societies world-wide. Sadly, it's part of the human condition. Shouldn't ending the fascism of cannabis prohibition be a prominent Democratic goal towards a better, more open, and tolerant society?