why can’t I find certain app in the app store on my iPad?

Without further information, the possibilities might be:

Not available as an iOS App.

Not available for an iPad.

Not available in your region.

Not compatible with your version of iOS.

Incorrect search criteria entered.

Not all apps are available in all countries.

What app?

What happens when you search for the app?

Any error messages?

Are you certain the app exists?