Accessibility too, they're on the high street unlike any other model game shop. Warmachine and Hordes, Infinity etc don't have that high street presence - Games Workshop do and they can get kids walking in off the street.
Many people could also name 5-10 Star Wars characters too. Personally, for the entertainment you get out of comicbooks compared to little plastic spacemen is massive, comicbooks are what $3.99 a "cheap" Warhammer set (10 men space marine tactical squad) is $40 not including the cost to put together and paint. Whereas comicbooks are likely to last a lot longer and let's say you read it 50 times the cost per read is less than 7 cents a read.