All is in the title. I have got a GT-I9000 (samsung galaxy S) and I do not understand why I cannot upgrade the android version ?
Why an android version is linked to a specific phone ?
First, you probably cannot update to Android 3.x as that version was only intended for tablets.
Second, no Android version is linked to a specific phone. But each phone needs specific adaptions due to different hardware elements used, so each Android version must be adapted to be usable on the device.
Third, for "How can I..." and "Where do I find..." and "When is it available...", please check the questions (and answers) already dealing with this:
Flashing new ROM will require to delete all your data from phone. And yes, by incorrect actions, you can easily break it.