Because the Democrats relied on control of the traditional media to promote a band of being the party of the little guy for decades.
For being a bunch of allegedly unhip and crusty boomers, somehow, the Republicans got a better handle on social media than the Dems did and their tailspin is a direct result of bringing their old weapons of the papers and TV talking heads to a battle they are woefully obsolete at winning as the enemy side has better ones.
The mask is slipping and showing the true face of the Dems as being incredibly elitist and intolerant and only paying lip service to the "little guy" while hoping they just die off so they don't have to rub elbows with them anymore because modern media can rip off the mask faster than they can put it back on, and they never invested in new media when they should have in the early 00's because they felt it would never grow to be more powerful than the CNNs and WaPos they had.
They controlled all the railroads and utilities on the game board, so they couldn't lose.... they never saw the Republicans buying Boardwalk and Park Place and putting up the hotels until it was too late.
I must say, I don't think it has anything to do with either party actually changing. They both have remained... mostly the same. Which is the problem (for Democrats).
The democrat party has long been run on a grease of hypocrisy, praising workers while encouraging immigration. Saying they are for the small man while aiding big corporations. Being for everyone, while being absolute elitist fucks. When they had control of the media and the narrative, they could downplay this and gaslight the nation. The internet and third party media right fucked them there.
Meanwhile, the republican party... also never changed. While there has been some downplaying of the religious angle over time, their core and even general views have remained largely static. This is for good or ill, depending on how much you like conservative policy. The thing is though, its not run on hypocrisy, its run on basic tenants and usually religious beliefs which can be softened (For example, on the topic of gay marriage) but which do not really change. So when the internet really got going along with third party media, the republicans came out looking like normal people compared to the democrats who were shown to be slime creatures.
Think about it, when you think of "Republican Hypocrisy", likely two things come to mind. "Gay Sex Scandals" and "RINOs". The former only effects the politician personally and isn't party wide, the latter is seen and acknowledged by the party itself as an issue to be solved. No real PARTY hypocrisy here, only personal hypocrisy.
Now think of "Democrat Hypocrisy". I'll wait a while, this might take a bit to complete the mental list.
Seriously though, several things probably immediately sprung to mind, and several more the longer you thought. And how many are spread throughout the party or, worse, are part of the party platform?