Most likely because when the original guys who created 40k from rogue trader created the marine legions all sat arounda table drinking..lots!
White scars are named because scars go white...lol
Ultramarine is a colour of blue, so make them blue!
Dark angels are a huge joke because of theprimark Lion'El Johnson, who was a homosexual poet a while back, so ther joke there is that all dark angels are gay.
20 is the number they could think up decent names of, simple lol