Why didn't the 2019 Oscars have a host?

Because so many people declined and Kevin Hart was ready before the controversy blow up and also they wanted to shorten the length.

Let's start with the timeline of events:

• 90th Academy Awards had the lowest rating so they asked Dwayne Johnson but,
  he couldn't due to his schedule:

  "Ah mahalo dude, I was their first choice to host this year, and my goal was to make it the most fun and entertaining Oscars ever. We all tried hard, but couldn't make it work since I'm shooting 'Jumanji'. Academy and I were super bummed but maybe one day down the road," he wrote. - The Economic Times

• Then Kevin Hart came into the picture, but the same old twitter drama happened where his past tweets caused controversy and he withdrew his name:

  “I have made the choice to step down from hosting this year’s Oscar’s,” he wrote on Twitter. “This is because I do not want to be a distraction on a night that should be celebrated by so many amazing talented artists. I sincerely apologize to the LGBTQ community for my insensitive words from my past.” - The New York Times

• Previous hosts have shown no interest:

  Previous Oscar hosts such as Seth MacFarlane, Ellen DeGeneres, Neil Patrick Harris, Chris Rock, and Jimmy Kimmel expressed no interest in hosting the show. - Wikipedia

And they decided to go hostless.

Also from Variety (key points):

The Academy had been considering welcoming Hart back if he had used the opportunity in an interview with Ellen DeGeneres to demonstrate contrition for his anti-gay jokes and recommit to hosting the show. But his sit-down with DeGeneres, a sympathetic friend, backfired when he was made out to be the victim of internet trolls and “haters” rather than the perpetrator of hateful remarks.

DeGeneres implied that the Academy wanted Hart back at all costs, to the point of quoting someone in the ranks admitting the organization may have “said the wrong thing” in discussing Hart’s exit.

While the Academy never commented on the veracity of the call, or whom it may have been with, one person said DeGeneres took the remarks out of context. A rep for DeGeneres had no comment.

Oscars producer Donna Gigliotti is also looking at ways to trim a show that has been accused of being interminable and bloated, insiders said. That may entail shaking up the usual monotonous stream of presenters calling up winners, though how exactly Gigliotti and team will tinker with that remains to be seen.

The good news, people familiar with Gigliotti’s plans said, is that key changes to this year’s ceremony have minimized the role of the host anyway. The new style and pacing of presenters will require less time from an emcee, one of the sources said.