When the metronomes are on the table, any back-and-forth force imbalances are resisted by the force from friction of table acting on the wood strip. Unless the motion/force is great enough, they do not move.

Now, by taking the wood off of the table and placing it on rollers (cans) there is a lot less friction to resist andy back-and-forth motion.

Unless acted on by an external force, the net sum of all of the back-and-forth of the metronomes, must be zero, so the wood strip moves in response to this net sum at every instant.

Since there is a small amount of friction on the rollers, the strip does not move exactly to the sum, and some energy is lost moment to moment. The energy loss causes the metronomes to change beat slightly, and this energy loss continues until they finally synchronize, eliminating any net force and further energy loss.

Without rolling friction of the cans, the wood strip would keep wobbling back and forth in response the instantaneous force differences, and the metronomes would not synchronize.

All in all, a very cool demonstration.

I hope this is right, and that it makes sense...