Well, off the top of my head, maybe for the same reason that I have a 7
year subscription to Green Lantern, 8 year subscription to Superman, and
close to a 10 year subscription to Fantastic Four built up?  In other words,
because you like the characters.

  I also have something like 5 years of Detective Comics, and 6 years of New
Avengers.  Sure, the quality goes up and down a bit here and there, but I
like to keep my favorites through the changes.

  I also don't like paying any more taxes than I have to, and the
subscriptions are tax free.  With retail, I'd have to not only pay for gas
getting there (road tax), but I'd also have to pay sales tax.  It would add
up to about a 10% overall tax for me, on an item that costs more than twice
as much, retail than subscription.

  So that tax comes out to about 20% of what I'm currently paying - not very
attractive.