I'd have to go with the answer that non Warhammer 40k fans, that is to say people who know what it is but dont actualy collect armies and so on, probably couldnt name half the chapters apart from the Ultramarines which is probably the most commonly known, as an example long before i got into it properly i knew the Ultramarines by name, this is probably one of the main development choices, as they want the game to appeal to the largest audiance possibly not just the die-hard fans, so they pick the most easily associatable chapter and more importantly the one that non fans can easily relate with knowing even a little bit about. I guess my point is that it wasnt a matter of picking a chapter that was right or wrong for the scenario but picking one that pretty much everyone knows.