Particularly in Voyager. They were either running extremely low early in the show or had a virtually infinite supply later in the show. No explanation given.

Which is particularly troublesome given the plot of the Equinox episodes. Since on that ship they were harvesting creates to extend their dilithium crystal supply allowing them to travel faster/less efficiently.

But even throughout those episodes they never discussed why Voyager rarely had issues with supply after the first couple of seasons. Just hand waved it away with Voyager having a bigger crew, like that magically solves it.

Voyager spent most of the first season looking for deuterium, so that they could keep the ship moving forward.

Presumably, at some point around episode ~15, someone in either engineering, or the writing team realized that it is the third most abundant element in the universe.

Well.... the most abundant element in the universe is "Nothingium." Just because hydrogen is common doesn't mean it's always around when you need it, especially in heavy form.

It's been a long time since I laughed out loud at an HN comment! Well played. Thank you for that.

It doesn't just have to be common, it has to be worth harvesting.

It could be that it took them this long to locate and get to such a supply, or that they had to repair / construct equipment that normally wouldn't be expected to be used.