As I see it there are two reasons why Marines are so popular. 1: they derive from the impression I get that Marines are the central masterpeice of the 40K universe; the epitome of the 41st millenium as GW sees it.
The entire gothic/medievil futurism is THE novel concept that GW has created. The central symbol of that gothic future is the knight of the 41st millenium, the space marine. They are a symbol that epitomises the 40K universe.
Everyone else, although original to some extent in most cases, are window dressing, and people for the space marines to fight against.
The other reason that they could be so popular is that, as a friend once pointed out, SM are really a testosterone army - Steroidally enhanced superhuman warriors clad in 1 foot thickness of plasteel and ceramite, weilding guns and swords.
Of course they appeal to those of us with Y chromosomes espeically (not exclusively of course) - the lack of subtlety, strength, eliteness, nobility (?), warrior monkishness of the space marine ideal is hardwired into our genetics and our society, in deep seated undercurrents merely muted since Homer's Odessy.
And since the majority of us are males who enjoy the idea of taking a bunch of little figures to war, of course we are going to get a disproportionatley high proportion of SM players.
GW knows the above two reasons, and tailors the hobby to suit the majority SM group. Not to do so would be to shoot itself in the foot. Not to say that the rest of us are unfairly treated, but if GW do not keep some degree of attention on the marines, their most vibrant recruitment of new players' intake, then they are not taking advantage of their strongest selling point. So they keep the marine range turning over - large, give them more vehicles, make their vehicles look better etc.
All in an appeal to the deep seated psyche of the majority, while epitomising the best originality that GW ever created.
It makes sense to me.