Q:

why don't I choose all versions for android app? 

When I created my application, I chose 1.0 for the minimum SDK and 4.3 for the target SDK, so my application works on all phones.   
But why would we choose (for example) a minimum SDK of 2.2? 
Is it about performance, or something like that? 

A:

Android is a system that is everytime changing, and that implements new features in each new version. 
When you develop an app, you will have to deal with :

Compliance for the user
Nice features for your application

For example, animations of clickable elements are not well implemented in Android 2.3.  So when you will have to develop an application, you will have to choose :

Compliance with a maximum users including 2.3 compliance
Develop a lot of nice animations, including on clickable elements

It's why sometimes you decide to develop an applications, you may decide to not include some versions of the SDK. 
And for having a clear view on how much users you would loose, you may want to go to :
Dashboards

