Why is Donald Trump such an awful public speaker?

Trump doesn't even try to be a good speaker.  Worse than that however he just stands there and lies and makes up nonsense.  He does that while praising himself and attacking entities he doesn't like with gross exaggerations and simpleminded hyperbole. 

Some things are just what they are. There are no clear cut answers for them. On the other hand Ronald Reagan was known a the "Great Communicator". He was radio broadcaster, a public speaker, governor and of course, president. He paid attention to the process of communicating, thus became good at it.

Trump doesn't seem to care about this except in one aspect, he cares about using sincere common language to relate to the public in general. In this he has succeeded.

But yet the majority of Americans love him

His style is to repeat so you will learn what he says. Most people retain nothing from a regular speech. The way Trump talks, you remember what he said. All of these controversial topics is simply him putting the ideas out so you will not be surprised. Trump has delivered speeches quite well, like the recent State of the Union was a masterpiece. Remember it was so good that Nancy tore up the official copy of the speech in a moment of anger and disgust. So, the back and forth stye is actually his repeating the ideas so that you and everyone else will know what he is saying. Not everyone is as smart as you.

No one elected him because he was a slick talker. As a nation, we had enough of that.

he's not a professional politician... the core reason he got elected in the first place?

think that's shallow and wrong?  well if you look back, all but two men who've held the Presidency since Nixon have postured themselves as "outsiders" who were "different than" their predecessors...

Carter was the humble country boy... different from all the "stuffy suits" of the Watergate Era..

Reagan was the straight talking favorite uncle...different from the "doom & gloom" politicians of the '70s...

Clinton was the smooth-talking boomer... different from the "dinosaur" politicians of the WW2 generation..

Bush 2.0 was the frat-boy boomer... different from the sensitive, "I feel your pain" "educated hippie" boomers...

Obama was the fresh new face... different from everyone that came before him..

Each new generation defines its choices in many ways by what it rejects in the previous one... Like it or not, many feel that all the past Presidents (including Obama) failed to deliver on their promises and aren't interested in their "pretty speeches" anymore...

...don't just look at Trump... look at the new generation of Democrats and what they're rejecting in their own party....

Because he is a zero disciplined rambler.

He is way better than stumbling, stuttering Obama. But so far, democrats complain about everything.

he wasn't elected to be a public speaker

Some people are like that. Some are good, some are bad.  Are you good or bad ?