There are a couple possibilities:
1) The perception you're creating with your ad, and the perception you're creating on the app store page may not be matching up. So the type of people that are being attracted to your ad, are not being attracted to your website. Meanwhile, people that would theoretically be attracted to your website presentation are never getting there because they're not attracted to your ad.

2) There may be some inherent flaw in your product that doesn't appeal to humans in general.

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Looking at your site, I'd say #2 is definitely one of the problems, and #1 may be as well (I'd have to see what your ad looks like). Here's a breakdown of the flaws of your page:

I. You have a typo in the first sentence of your website, "You can order for groceries". That doesn't instill confidence, and actually makes it seem like it might be some kind of phishing scam.

II. Let's say I want to actually use your service. There is _no_ apparent way to do that. Your main button says, "GET REWARD", which A) has nothing to do with, "I want to sign up for your delivery service", and B) makes it seem even more like some kind of phishing scam.

III. There is no credible indication that you actually have anything behind the curtain. It seems like it's just a way to collect email addresses and phone numbers, and that there is no actual service when people sign up.

There are lots of existing services like yours, from reputed services. Nobody will want to use something that both looks like a scam and has no reputation. If you'd like advice on how to work around the problems listed I'd be happy to help out.