The Republican Party is (and always has been) more ethnically homogenous than the Democratic Party. Democrats have to cobble together a coalition of white and nonwhite voters, religious and secular. That heterogeneity causes voters to demand representatives who cater to their specific identities; Democrats from rural red states carve out more conservative political identities, while those from college towns or heavily nonwhite cities will differ from each other in their outlook. The Republicans are overwhelmingly the party of white cultural traditionalists, and more naturally coalesce around a shared agenda.
That has been true for decades, of course. Two other factors have become true only in the last decade or two. The Republican coalition has grown increasingly authoritarian in its personality outlook. The concept of an authoritarian personality is controversial among academics, and the definition of the term has been contested and changed over time. But it has gained renewed salience and explanatory power during the era of a president who seems to wake up every day looking for new ways to bear out authoritarian personality theory, by disregarding the rule of law, excoriating minorities or news media as enemies of the state, and praising his followers for their mindless loyalty. It is implausible to imagine in the current moment that a party that elected Barack Obama could behave in the same way as the party that elected Trump.
The final reason is the rise of a party-controlled news media within the GOP.