Why I got into SG and why I love them today are two different things.
I cut my teeth on Rogue Trader, which I still think is a great game, and very shortly after got into Space Marine first edition and Space Hulk first edition. The initial draw was clear - 40K required a book and multiple packs of figures, while the other two were complete games in a box.
Probably the theme that has kept me in these games from the start to now is the 'cinema' of it all. I love the idea of sweeping combats in Epic with air support, giant Titans, thousands of troops, scores of tanks.... or grande space operations or whatever. To me, the SGs encompass a lot more spectacle than the core 40K/WHFB games.
Also, because they are not a core product, they are not afraid to be disliked. 40K and WHFB have to appeal to all gamers at all times, because they are the mainstream products. The SGs are unashamedly targeted at certain types of players, and unappologetic about it. Lots of people dont like them for a variety of reasons, and thats ok, but it also makes them stronger to me in their theme and focus.
A clear example is 40K which is a skirmish game.... with tanks and aircraft and Knight Titans.... because it needs to appeal to people who want a variety of things in their games, and sell the minis.
Finally, because of their ability to be free for what they are, the SGs can be targeted at a gamer who isnt so interested in 'buckets of dice' (for example) and I find give a much more tactical, flexible and rewarding game, which is vastly more replayable because of it.