Mars is a dead planet. We made it that way.
We had atmosphere recyclers, and were working on ways to extract it from the rust rocks all around. But even with supply shots every few weeks sped up by low cost launching, it remained that we were just a few malfunctions away from disaster.
A number of the suicides were caused by people wondering about the what ifs. What if it had been one international effort rather than several nations with separate colonies? What if we had been able to pool every thing immediately? What if a few had just stepped forward to make a sacrifice, or we had drawn lots for it? Would a little bit of dignity and an acceptance of death been enough?
Too late now.
Short version - we turned on each other. Raided the other settlements to ensure we had spares for the oxygen scrubbers. Misappropriated rock hammers and mining lasers to kill each other. Maybe we could have stopped once the population was at a fairly sustainable level, but the esprit de corps, and the call for revenge was too strong. The victims were people we had seen every single day for years, specifically chosen because we couldn't bear to kill them - so of course we couldn't just accept one of our team mates was dead and move on.
There still isn't much word on why all contact was lost, or what those alien ships are - but unless there is an outright zombie apocalypse back there, it can't be worse then living here.