I think it's a lot less clear what the societal benefits of forcing copyrights to expire are. For patents, it's much more clear -- a patent prevents others from copying the invention, so its expiration allows other to use that invention for economically interesting purposes, which have a net societal benefit. So on the one hand, the temporary monopoly allows the inventor to profit from their invention, and costs others their ability to exploit it, and after a certain amount of time, those needs are reversed.

For copyrights, in the purest form, what do you really get from letting it expire? The only really tangible benefit is to allow people to obtain the work without having to pay anyone, and (theoretically) make it easier to preserve and distribute orphaned works without the permission of the creator. It seems like as a lawmaker, it's really not that hard to rationalize saying "I can pass this legislation, extending copyright, or I can directly cause Disney to forfeit X million dollars in revenue".

At least with a patent expiration, you can counterweight that by saying "I can extend the patent, and Boeing continues to get X dollars, or I can allow the patent to expire, and other companies get to use that invention to make Y dollars." The tradeoff is a little clearer.

I would love to see better rules for copyrights moving to the public domain, not so much for the sake of Mickey Mouse, but rather for obscure and orphaned works, as well as simply decreasing the cost for the consumer to improve accessibility. But it's very easy in my mind to see why this is unlikely to happen.