I think it's something else. My area like most is 40K, but there's a strong fantasy contingent. That being said, I used to think along the lines of 40K is more for kids/young adults, and fantasy is for adults. I'm an example - started out with Fantasy, liked the look of 40K, switched to 40k, before stopping the hobby (moving to uni, money reasons etc).
Now nearly 6/7 years later I'm 23 and loving fantasy again and find the tactical approach, the vastly different armies and army rosters of each race far more interesting and diverse. Say what you want about the games, but 40K certainly has the certain-lists-smash-tournament kind of armies - remember the space marine drop pods lists?! - but fantasy has a much, much larger amount of competitive lists. Yes, you need mages to be competitive, but that's like saying you need troops to be competitive. Of course you do, it's a given. But it's not the be-all-end-all. If you spend all your points on mages, they're going to die. Badly. They need support troops. Fantasy is the game of a perfect army roster balance. Core, heavy hitters, troublemakers, mages, artillery, monsters, monster hunters, the list goes on.
So why is 40K more popular? I think the two reasons stated are pretty accurate; easier learning curve, less models per army equals less cost. But for me, the main overall reason is what I'd call the 'instant action'. Things die in 40K, and they die quickly.
Think about it - Fantasy is the game of tactics. Movement, the right charge, flanking, breaking, running down. It comes down to a very big game of rock paper scissors. Everything in the game has a counter, a weakness, and the meat of it is to exploit your opponents weaknesses without allowing your own to be hit. Your army is made up of rock-paper-scissors units, and its about matching them with the right targets, and throw in some luck.
40K on the other hand, whilst I'm not for a second disputing the tactics and some variance in lists, I'm a fan of both games after all, is more forgiving. You learn quicker, because there's less to use, and certain weapons can kill most things. You can make bad decisions and lose but make similar bad decisions in fantasy and more often than not you'll be massacred.
And as I said, things die much quicker. On turn one in 40K, you're not so concerned with moving your troops the perfect charge distances, you're not so bothered with allowing your flanks exposed, you're not trying to move a battle line. Your guys go forward, or they hold steady for the most part. But they shoot, they shoot a lot, and something often dies. It's quicker action, less foreplay. Guys run forward with guns and tanks and blow stuff up. Therefore it appeals the action gamer, and the kids who want to see something happen straight away.
But for those who like the foreplay of fantasy the real (and for me larger) reward is watching your epic plan of battle go perfect. And it's damn hilarious when it doesn't and your mage blows up, your hellcannon eats its crew and your chaos warriors flee from skaven. That's the luck of the game, you try to minimise the luck, but it's what makes it unpredictable and fun. It's about risk versus reward and tactics versus luck. For me, it's a fine and dandy balance. It's a poor metaphor, but if Fantasy is Chess, then 40K is closer to checkers.
Though I've noticed far more kids (by kids I mean below the age of say, 21) playing Fantasy at my local GW and clubs. It's a cool sight. I think most fantasy players fall into two categories: you have the guys who like the tactical aspects and greater depth of play, and you have the guys who like the fantasy setting over the sci-fi.
(Though I'm going to be awkward and say I think the 40K universe is far more badass. I love the fluff more, but fantasy has its own and great stories.)
My sons, the galaxy is burning.
We all bear witness to a final truth - our way is not the way of the Imperium