iTunes displays ads based on the information it collects about you (personal preferences, apps, searches, usage data…), but if you are wary about your privacy then you'll be happy to learn that Apple provides a simple solution to opt-out of interest-based ads.
How To Limit Ad Tracking in iTunes
Open iTunes, head to the Store menu > Sign-in and sign in with your Apple ID. Next, click on the Edit menu > Preferences > Store. Tick the Limit Ad Tracking checkbox:
Click on OK to apply the new setting and close the Preferences window.