Because you have a 4G iPod
As Apple said in June, iOS 7 is not compatible with the 4G iPod.
All the iOS devices compatible (iPhone 4 and later, iPad 2 and later, and 5G iPod touch) with iOS 7 have at least 512 MB of memory (RAM). The 4G iPod only has 256 MB
To download a compatible version:
Starting when iOS 7 was released, Apple now allows downloading the last compatible version of some apps (iOS 4.2.1 and later only)
App Store: Downloading Older Versions of Apps on iOS - Apple Club
App Store: Install the latest compatible version of an app
You first have to download the non-compatible version on your computer. Then when you try to purchase the version on your iPod you will be offered a compatible version if one exists.