Call it an oversight, or call it a deliberate thematic choice.
At one point, the Legions were supposed to be 10,000-strong. The nine loyal Legions somehow amounted to a thousand Chapters with a manpower of 1,000,000 Space Marines by M41.
The fluff evolved, and the Legions became much larger. The average Legion numbered between 100,000-120,000 Space Marines. Despite this increase, the manpower of the Adeptus Astartes in M41 did not change.
This was not simply because the Imperium incorrectly thought the Legions numbered 10,000 Space Marines during the Great Crusade when in fact they numbered 100,000-120,000. The writing team themselves believed this, until it was decided the Legions would be larger.
Who knows why there are only 1,000,000 Space Marines. I don't buy the idea that this is because the process of making one HAS to be so incredibly difficult. The Ultramarines have been proving for ten millennia now that you don't need a murderous environment to get high-quality recruits. At the opposite side of the spectrum, several largely undeveloped/technologically backward Legion home worlds proved during the course of the Great Crusade that they could maintain the manpower of a Legion that - depending on the timeline of the hobby - was 10-120 times bigger than a normal Chapter.
Bottom line, I'm willing to accept that the Chapters make the process difficult on themselves because (A) it makes them feel elite and (B) the Imperium's cap on their manpower means they can afford to be much more selective. I don't buy the notion that they HAVE to find that "one in a million" twelve year-old that can successfully hunt dinosaurs, though.
Where genetic compatibility is concerned, I honestly think that's blown out of proportion to a degree. Caliban was able to crank out about four thousand Space Marines every two years - albeit under optimized circumstances. You have to remember, though, that Caliban sported a feudal society in a Death World environment. If genetic compatibility requirements were such that only a minority of even un-mutated humans could meet them, there's no way a planet with such a small population could maintain recruitment needs. More likely, genetic compatibility came down to prospective candidates simply not suffering from mutation.
That correlates with what we see in "Angels of Darkness": Boreas' recruiting trip consists of visiting a tribe - of no size worth mentioning - that trots out twenty teenagers. Boreas picks the top three of the crop and checks to make sure that they're compatible. Surprise-surprise, two out of three are. That third one? We don't even know that he wasn't compatible. He just happened to have second thoughts about joining after seeing Brother Nestor dig through the first candidate's intestines as part of the "examination process."
Last edited by Phoebus; 04-30-14 at 03:51 PM.