Star is the most common mode of rating used - Is there any specific
reason for this? Or is it just because of a convention?
Well, there isn't any inherent meaning to a star symbol that relates it to a rating system (that I can think of). That means its use is a matter of convention. It is a convention that has existed for over a century: Wikipedia gives some examples of early use.
I wasn't able to find anything on the reason this character was selected, but my guess is that it was because of the limits of early 20th century typesetting/typewriters. There weren't that many symbols to choose from back then.
Having said that, don't overlook the importance of convention. While the initial reasons for the convention may have been arbitrary or long obsolete, that really doesn't matter. What does matter is that the convention is now widely recognized.
You don't have to use stars, but if you do, everyone will instantly recognize the stars as a rating. If you pick something else, you will have to work harder to make sure users understand it.