One of the prominent themes that has run through science fiction since its beginning has been colonization: the settlement of a fresh, new world, and the testing of men and women against its unexpected challenges. I can understand the popularity of the concept; who wouldn't want the opportunity to be in at the ground floor of an entire world, to stamp their influence on a new society from the very beginning? Sure, it would be difficult, but that's never stopped humanity before.
Problems occur when the creator doesn't give enough thought to those unexpected challenges. Settlement of a new planet is a difficult, expensive business; it's easy to forget that exoplanets would be the result of a completely alien development process: no matter how much they might look like British Columbia, if their life is based on dextro rather than levo amino acids, you might as well have landed in Death Valley. This seems to be a relatively new concept; Mass Effect was the first sf series I encountered that addressed the issues of chirality. Earlier settings, like the Co-Dominium or Battletech universes, seem to take it as a given that Earthcompatible planets will be numerous and settling them won't be much more difficult than going down the Oregon Trail. Even if you go the route of having planets terraformed in Earth's image by Precursors, as I've been leaning toward, there's still the issue of evolution running in different directions afterward; the first wave of colonists might well have some difficulties against roving packs of Tyrannosaurus superbus, for instance.
It wouldn't be good if they all got eaten. After all, the core requirement of a society is people, and if you're a society that wants to progress and improve, you need enough people to support specialists. Below a certain population level, the necessities of survival are such that more complex things can't be maintained - consider the Aboriginal inhabitants of Tasmania before European contact, who had lost mainland technologies like bone tools and fishing. There are plenty of instances throughout the historical record where a society numbering in the hundreds failed and was swallowed up completely.
I've been thinking about this recently because of Stargate Universe; in the latest episode, the Destiny crew encounters members of a society founded by themselves - or, rather, their temporal duplicates, sent two thousand years into the past. Aside from the Eternal English, it runs headlong to the problem of numbers - with effectively nothing more than the clothes on their backs, on a completely alien world that nevertheless looks just like British Columbia, the entire story is based on the premise that less than eighty people managed to survive, avoid falling into pretechnological darkness, and ultimately build an advanced and sprawling civilization numbering in the millions in the course of two millennia. That last one isn't that much of an issue; a seed population of seventy-five doubling itself after every generation would result in a population of 2,457,600 after fifteen generations; call it three hundred years, though you're unlikely to be able to sustain that kind of growth rate for long without any social or technological infrastructure. Nevertheless, the episode implies that the Destiny crew not only survived but were able to establish a respectable garment industry after only ten years.
The ultimate distillation of this concept is the Shaggy God story - a tired, worn-out sort of yarn where a male and female astronaut (not necessarily, but frequently) are marooned on an habitable, empty, paradisical world and are - dun dun dun - Adam and Eve! I can understand one reason why editors hate to see this sort of story turn up in their slush piles again and again; it is not thought through. Societies are not easy things to start. Two people is not enough to cut it. Even if they're lucky enough to survive without dying in a thunderstorm or flood or wildfire or from starvation or cold - or the cold, since ordinary things become society killers when you can fit that society into a Geo Metro with room to spare - there's the problem of insufficient genetic diversity. Do you really want to found a civilization on the concept of inbreeding?
Settlement needs people; it's as simple as that. I can understand why creators might want to start with small seed groups; it gives us a chance to know most, if not all, of the members as individuals, to root for them singly as well as collectively, and to see them overcome the vast challenges in their way. But when that comes at the expense of believability, it's not something I can really stand by.