Because Bitcoin had to be available for commercial purposes.
Well that probably enabled CSW Faketoshi to claim the ownership of Bitcoin whitepaper, that was also released with MIT license if I am not mistaken.
I doubt CSW know or care about license on Bitcoin whitepaper before he decide to act as faketoshi.
There is also Satoshi's identity to think about. Copyleft is tied to an individual who can exercise some rights. Since Satoshi wanted to remain anonymous and not exercise any rights, he used MIT. Copyleft would have caused a more difficult legal defense against several Satoshi claimants, whereas MIT allows that to be foregone entirely (hopefully).
There is also public-domain license he (or they) could use back then for Bitcoin, and there is no ownership issues with this license.
It's not that easy. Some people argue public domain isn't really open source/software license. GNU even recommend you to use CC0 in case legal system doesn't acknowledge "being public domain"[1]
[1]
https://www.gnu.org/licenses/license-list.html#PublicDomain