When I first buy this game from Steam , it was on Early Access , it was on alpha or even before alpha . And I was saw that application name . But I was ignored it because it was really early stages of the development . Now the game is released but still application name is same . Why ? Is it too hard to change that application name ? I am wondering why still it's in same name .

It probably wasn't considered worth anyone's while to change it. The older name turns up in logs too, with lines like

Divinity Original Sin, which was originally planned for release in 2012, and was named Project E, was going to be called Eyes of a Child, hence the application name. Why it was never changed is anybody's guess, but it's hardly worth worrying about -- I bet that was the last thing on the developers' minds.