i learned how to play
40k when i was 12 by my brother teaching me. won my first
rtt when i was 13 just a bit after the first blood angels codex came out. i then learned to play fantasy. took me 4 years and at least 10 tournaments to at least top 3. i quit
40k in 04 and just played fantasy untill 2011 and just recently got back in it. learned most of the main rules in two games without looking at the main rule book.
fantasy is a much different game. to me
40k is a simpler game. it was so easy to learn again. it plays faster then fantasy and its sci-fi. people will always like sci-fi then fantasy based settings.
40k is also the game that is marketed more by
GW to the masses. video games like dawn of war and space marine were marketed alot more then the online fantasy mmorpg and the old playstation game shadow of the horned rat.
and like i said before the ruleset is easier to learn in
40k. i recently tried to show a
40k player how to play fantasy. it was like he was learning a new language. the tactics and thinking involved in every action of fantasy takes alot more out of you then say
40k when you just go forward and shoot people.
also people like to win. in 8th edtion fantasy any army has a chance of winning and beating everyone else( even wood elves, though you have to be a great general). their isnt those specific armies that win a tournament or list that win alot. there are suggestive builds and some units you will always see in some armies in fantasy but they are not as bad as the dragiowing or venom spam.in
40k people have push start armies like grey knights, space wolves, dark eldar, blood angels, necrons or anything that matt ward writes

. in these armies you dont have to be a great general to win or beat out other armies.