Question: In the near future the demand for resources only rises due to both a ever growing population, energy and resource crises, and rampant consumerism. Peak oil has been reached, scarce resources like platinum, silver, and other rare metals have been depleted on the surface, and the world is beginning to feel the more drastic effects of climate change such as mass extinctions of animals and strains of crops. In this rather bleak period the major mining companies looked to the seas to solve the mineral requirements of earth. While mining the seas isn't a easy affair the ocean's mineral wealth is virtually untapped.

However instead of using drones to mine both ores and the methane ice the big companies used mainly human labor (with drone assistance), but why? What benefit would human laborers in deep sea rigs have over a aquatic drone?

Note:

The population is fed mainly by cultured meat and indoor+vertical farming hence why the population is expanding even though the climate looks bleak.

Asteroid mining isn't considered a good alternative for earth due to the transportation of the goods back to earth. However if necessary the industry does exist (mainly for the initial space colonies)

The mines would be accessed by either mining ships or for the larger mines maglev trains connected to large underwater bases (with the tunnels dug under the ocean's surface)

Answer: 
The mining companies are being subsidized
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Drones would have been better value for money, but the government subsidizes human labor.
This is done to keep the population (especially the men) occupied and also safely locked away, not giving them a chance to sit around talking rebellion.

This is also the reason space mining is not favored: there are too many places to run and hide once the tech for space colonization exists.
If you can tolerate some suspension of disbelief, this could still be plausible.

Perhaps...

* Your drones cannot be reliably remote-controlled at great depths?
* Your society needs a reason to keep the growth of the lower classes in check? Compulsory military/mining service fulfills this purpose.
* Drones are simply too expensive? Surely their construction consumes a disproportionate amount of the rare minerals they exist to mine?
* Espionage/sabotage-- foreign corporations hijack/destroy each others' drones? Unless you intend on fully automating undersea warfare and wasting what's left of the resources having robots fling torpedoes at each other, humans are more expendable.
* Putting people to work keeps them out of trouble. You can automate all unskilled labor, but now you have a bunch of bored/desperate people surface-side. Crime increases under these conditions.