it's stupid play store compatibility filter. It's always wrong, tends to allow actually incompatible stuff to work and cause issues, or falsely flags specific devices it hates as not possible to work. I haven't depended on an app store since Android Market EOL'd, and i second trying to use the APK file from a site such as apkmirror or apkpure. It's been my go-to for years, and best yet, no surprise forced updates.
You can also choose which version you like. If I actually wanted Instagram, I'd use the last version that sported the old icon as I hate flat ui design anyway, and I hear the old app still works.
If you feel you need an appstore, try an alternative such as Aptoide. That's about as close to Android Market as you're gonna get today. There is also SlideMe if you're into the era of Android 2.x which was back when it was truly open and free of Google, or F-droid for only open-source apps.