Here are some reasons:
1) Issue #1 only has a print run of 7,266 copies.
2) WD is one of the top TV shows in the country. The ratings increase pretty much every season. Season 6 is starting in October and I would be very surprised if the show ends before season 12.
3) Fear the Walking Dead will be starting in 3 weeks and has already been renewed for season 2. This show should be a huge success. Even if they lose half the audience that watches the Walking Dead, which they won't, it will still have 10 million people watch it every week. There is a strong possibility that if this show becomes a hit they can start a third walking dead show. Bottom Line: The Walking Dead is HOT. I can't see it going down in popularity for many years.
4) The Walking Dead will eventually have a movie or two made. This will probably happen after the TV is cancelled.
5) The Walking Dead is bringing non comic readers into comics. I work in a large building and I know of at least 10 people that buy the walking dead comic book and TP even though they don't collect or buy any other comics. The Walking Dead is brining mainstream people into our hobby. It's only natural that some of these people will want to purchase the #1 issue. You can't compare the Walking Dead to any other comic currently being published.
6) NM #98 and SM #300 are two modern/copper comics that have print runs over 100,000. They also have many copies graded 9.8 and the prices keep going up. If these two comics keep going up with their huge print run I don't think the Walking Dead with their 7,266 print run has that much to worry about.
I'm sure there are many more reasons. This is what I came up with during my 15 minute break at work.
AMEN to all of this. Walking Dead got me back into comics in 2012. Hadn't purchased a single book since 1993 growing up. After TWD it lead me to start purchasing other key books such as NM 98 (First non walking dead book I purchased ) Demand outweighs supply and it is one of those book lines like TMNT that will have staying power for many years and many tv show / movie reboots, graphic novels, comic spinoffs , etc.