There are different causes to this issue. For example, the developer didn't include your phone in the compatibility list. Even if your phone has Android operating system, it's marked as being incompatible with the application and because of that you can't install it.
There is another way if you can't install it through the Play Store. The APK can be downloaded through various websites on Google. Download the APK, transfer it to your phone then initiate the installation procedure.
Warning: It's not RECOMMENDED installing apps from third party websites.