A change of doctrine shifting the use of machineguns and demanding more mobility. Water-cooled machineguns as we saw in WW1 were intended for long periods of sustained fire from relatively static positions. The advantage of being able to fire nearly indefinitely allowed them to perform both direct fire - shooting directly at a target - and indirect fire - "lobbing" rounds in an arcing trajectory to create a beaten zone at long ranges. That second point is the important one - sustained long-range fire over the heads of closer troops could be used to create a beaten zone to deny an area to the enemy. If that sounds like an unusual role for machineguns, that's largely because this kind of indirect fire has since been taken over by light mortars.

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The big problem with water-cooled machineguns is their weight and logistical requirements. Water-cooled guns are able to fire for extended periods without stopping, but they're very heavy require a source of water. The water requirement seems trivial, but it was enough for the French army, which had lots of obligations in arid regions, to decide to adopt an air-cooled heavy machinegun prior to WW1. At the same time, warfare shifted towards more mobile combat. Already by the end of WW1 countries were coming to realize that a lighter, more mobile machineguns were needed to support offensive operations, and the Light Machinegun that took shape over the interwar period would trade off absolute sustained fire capabilities for mobility. As light machineguns were refined, easily removeable and replaceable barrels became commonplace as a way to help the guns fire for longer periods without a water-cooled barrel. Since WW2, that's largely become the standard for light machineguns - barrels are designed to be easily swappable in combat and machinegun teams would have multiple spares to allow for teams to sustain long periods of sustained fire without overheating the guns.