They don't make enough off of subscriptions to make publishing worthwhile and if they give it away for free, the advertisers don't value the customers as highly. That's why free magazines like MacWeek and PCWeek forced you to take surveys to qualify for a free subscription back in the day.
Ha ha, I've also cursed Wired for all their ads, particularly at the beginning of the magazine. Them and the subscription card, though at least it can function as a bookmark.