I realize that this is a hot take, less than twenty-four hours later at the time of posting, but is any technical detail known about why the core engines failed to restart, causing the crash?
The answer given by Musk in the post launch conference call was that they ran out of the TEA-TEB igniter fluid, which is used to start or restart the engines.
On the ground, the TEA-TEB is externally provided for launch to start all 27 engines.
For landing attempts, only 3 engines are equipped with TEA-TEB to allow restarting.
It is a little hard to hear, but Musk seemed to say they ran out of Boron, which would imply the TEB specifically.
SpaceX just tested a three engine landing attempt that worked (sort of, there was no ASDS underneath it to land on, but it landed soft enough that they found the booster floating on its side) so they know how to do it. Likely there is some difference with the Falcon Heavy core stage and the standard Falcon 9 that may account for this issue.
I believe that there was a failure in procedures on the ground to put Tea mixture in the core booster. It ignites instantly so I don't believe they ran out for the core but not the two other boosters.