Because Bitcoin had to be available for commercial purposes. It was intended as a currency, which meant that businesses must be left unconstrained for how they incorporate it. Otherwise you would have seen a lot more lawsuits related to it -- a simple one would be early developers who thought their contributions should be paid back in the future after projects took off that are secondary to their work.

MIT, instead of copyleft, was the best option for being open while allowing people to develop their own projects. People have a choice to not use closed source projects -- or to use them, if they fit within their philosophy.

Yes, the MIT license allows all sorts of shenanigans, but it allows businesses to develop. If businesses had to concern themselves with copyleft, then it would create even worse shenanigans, like contributors of minor changes fighting for a share after projects became much larger.