How did Godzilla become the last of his kind?

Two words. 

Natural selection.

Because two godzillas on the same screen will destroy the universe.

jks aside I was thinking that to.

Cannablism, fighting over mates and/or territory, predators, change in climate, lack of food. There a lot of reason, hell everything I just mentioned are more or less what killed off the dinosaurs. The idea of these factors killing off godzilla's species isn't that far fetched.

considering how brutal godzilla can be, they all prbably did just kill eachother off. pherhaps over what maggleshackle mentioned. i think that combined with the presence of the mutos makes the most sence. i definetly do not think it was only the mutos.

Yeah, MaggleShackel and GodzillaHimself seem to be on a sensible track.

I think that, maybe, just maybe, Godzilla isn't truly the last of his kind. There are others, but they are dormant, and have chosen simply to remain hidden.

More far-fetched than anything, but it's at least feasible.

Godzilla has always been "the last of his kind."

The first Godzilla was actually meant to be a stand alone film just as this one is, but the success fast tracked a sequel to try and duplicate the success. In order to do it, they had to tweak the lore of the first movie to allow for another Godzilla, but the original Gojira was supposed to be a singular entity all on his own without any others.

Dr. Yamane said that if mandkind was foolish enough, they might create another Godzilla. Does this mean another Godzilla exactly? No. It means that they might create another creature, substance, or process that could cause the same kind of damage as Godzilla. It could be in the form of a Kaiju, or it could be something entirely different like a weapon humans create themselves. Ultimately, the first movie was supposed to stand alone, and thankfully it does.

But Godzilla was always meant to be the last of his kind. It's part of his design. It doesn't necessarily need an explanation, but if there is one then it likely has more to do with the facts surrounding his existence than any animalistic behavior.

Being a creature that survives on radiation, the lacking of natural radiation in the world after a certain time period in Earth's history would be the primary source for his kind's decline. Without his kind's natural energy source in abundance to suppor them all, they die off just as any kind of creature that loses its natural food source.

I truly do hope that this detail is never explained in full. I like imagining what the possibilities might be of at least a few individuals being leftover, or vice-versa and how he truly is alone. In fact, perhaps isolation from each other is what would eventually drive the species to technical extinction. How lonely would Godzilla be then? It's clear that he's no mere animal. He's bigger than that, figuratively and literally. If he does feel emotion, then millions of years of being by his lonesome must be depressing.