but I'm very curious to know that why Bitcoin core wasn't launched in public domain?
I think the Creative Commons Zero license would be even more permissive and easy to use than the MIT one, and that license doesn't require the attribution to the original creator. If the reason was permissiveness then both public domain, and CC0 license would provide more freedom.
MIT license is more common in software, while licenses such as the CC0 tend to be more in creative works. In addition, the MIT license requires a copyright notice in each of its future redistributions and is therefore better for the programmer, especially if he is developing the code for free or being paid indirectly as it happens with some Bitcoin core developers.
You have to distinguish between Bitcoin, which is a protocol, and Bitcoin core, which is a full node program.