How does Google Play work out if an app is compatible with my device?
When developers make apps, they have to specify any hardware/software requirements in a file called the androidmanifest.xml file. 

These can be hardware or software requirements, such as if the app needs GPS, or telephony features - or if it will only work on a certain Android version.

When you use Google Play, it sends your device's 'features' to the server, and then filters apps accordingly.

The most likely reason it is deemed incompatible by Google Play is that it is set to require a certain screen resolution - or maybe portrait/landscape mode defaults.

Some tablets have a low resolution, and also tell google play that they cannot go into portrait mode.

It could also be, your tablet being generic, that it does not have the required GPS or sensors that the manifiest file says it needs, and the market is stopping you from installing because of that.

I shall find out what the instagram app needs (look at it manifest), and edit this answer for you.

EDIT:

According to the Instagram manifest, the only 'required' item is the glEs version - minimum 131072. It could also be because of your tablets country/carrier setting.

Twitter needs a minimum of Android 2.1, and may also need a Vibrator, as it uses the vibrate permission.