The video ends with a brief second where the cyclist explains what prevented even higher speed. I make out: "too much vibration...", "knees rubbing...", "unable to continue increasing power output". Would love to hear more about this.
Note the video was from last year when they failed to set the record. This year they've done it. They probably will release a video of this year's efforts eventually.
I believe it is because the vehicle is very aerodynamically unstable. When you ride a normal upright bike, it will fall over unless you actively spend a little effort balancing it. Here is a video of a crash: https://www.youtube.com/watch?v=i5Dapy1xUq0 Note how it wobbles to the right when he pushes the right pedal, then to the left when the left pedal is pushed.