What can cause an application to be available for sale elsewhere and not in the USA?
You will want to review the regional settings for that SKU by logging into iTunes Connect.

This is documented at Apple's iTunes Connect Developer Guide

Scroll down to the part where it describes the following:


  Setting Territory Rights, Pricing and Availability
  
  On this page, you:
  
  
  Set the date when you want your app to go live on the App Store
  Schedule price tier changes for the future (by setting a price tier effective date and a price tier end date)
  (iOS only) Offer a discount to educational institutions buying in bulk
  Select the territories where you would like to make your app available (do nothing to have your app available worldwide)
  
  
  If you need assistance as you’re filling out the page, select the
  appropriate Help button (?) next to the field or refer to Table 7-2
  for the property descriptions. You can modify these settings at any
  time, including after the app is available on the App Store.


Also, before calling for help, be sure you didn't remove it from sale by setting a future availability date for that SKU:


  Important: Your app’s availability date ultimately determines if your app is live on the App Store or not. If your availability date is
  set to a date in the future, even if your app is Ready for Sale, it
  will not be live in the App Store until your availability date
  approaches.


I would assume there's a problem with your account in the case where you can't fix this immediately or the help doesn't work. In that instance, contact Apple directly at Developer Support for iTunes Connect