number of reasons make it much harder to advertise to the app store.
there's so many steps. you go from your marketing iPhone site, to the app store page of you app, and then you have the chance to buy it. thats at least one, if not two more steps. why can't you have a nice big "buy now" button on your website?
it's a black box. you can't see what works, you have no way of knowing if 75% of your tweets convert into a sale, and only 10% via fusionads.net - its not possible to tell.
iTunes or iPhone only. Good luck buying an app if you're at work, or on any computer where you don't have your iPhone set up on. Instantly preventing a) apps over 10MB from being installed, b) the casual news browsing "oh that looks like a cool app" you do whilst at work.
no refunds or trials. it prevents users from downloading ~$5 applications just because - you're not allowed to make apps that "explode" after 30 days. If you give away free trial apps thats fine, but a ton more work for the developer.
and lets ignore the fact you might get your application held up for weeks after you've been told it'll launch 1st August - after you've got all your press ready and such, they might just decide to not release it then. no reason why.
note: we're launching an iPhone app, and i agree with you - its all about the marketing, and thats why I've switched from dev to marketing for the next ~2 months before launch to get our iPhone app rocking.