Juries require fairly fixed starting and ending times so that the juries can have predictable times of day within which to get their other business or family affairs done. They need more and longer breaks during the day, to keep their attention. They may need to hear the evidence in a particular order to be able to avoid confusion. They are seldom able to go two hours late in order to finish a witness. Evidentiary objections are more important with juries than in trials to an arbitrator or judge, so there are often sidebar conferences while the jury has nothing to do.
Bench trials and trials to an arbitrator can proceed relatively seamlessly. Evidence that would be problematic to put in front of a jury may freely be put before an arbitrator or judge because they are much less likely than a jury to be misled. There are fewer objections, and the objections are less likely to require sidebars from the witness. The parties and the arbitrator or judge can agree to start early, finish late, or both to get a witness done and off the stand. They can agree to take evidence out of order for the convenience of parties and witnesses, and it does not affect the ruling.
The parties and the arbitrator or judge can agree to take a shorter lunch break, and take fewer breaks within the day, to finish a hearing or one phase of the hearing.
And, of course, no time is spent with jury selection, voir dire, and challenges.
Simply put, arbitration can move much more quickly than a jury trial can reasonably be expected to move.