Recently I have decided to move my site form a .rs domain to a .com one. With that I also upgraded my software (OpenCart 1.5 to 2.0). Both the .com and .rs sites are currently operational. What I have noticed that in a matter of days Google has indexed the .com version of the site and in most cases is ranking it higher than the .rs one that has been operational for some two years.

I am confused to why this has happened, is it related to the domain, or does it have to do with the new software? I am pretty new to SEO and this has me puzzled.