because you can get them everywhere and they are not what we call "fancy" nor intelligent. all they like to to is eat and put long strings in your tank...they mostly all do the last one..but they are not that appealing or expensive.
yes yours is a Common, it has a potential to get over 30 inches long and is better suited to a pond over 2K gallons.
actually they are quite intelligent, and many people with the ability to keep them find them very appealing. they can be hand fed, eat from your fingertips, come to the top of the tank for a petting, and can develop some really amazing coloration.
"common" is just what most people call them because they are the most often seen, almost everywhere. other people with more background call them by the scientific names, which are more appropriate. Commons only reach 20-24", and will be fine in a tank 2-3ft. wide with at least 150g's. Saying they need a pond is a bit of overkill.