Disney gets blamed a lot for this, but I don't really buy it. The copyright extension in '76 brought America into the same copyright duration as stipulated by the Berne Convention of 1886 (although the US would not sign on until a decade later), and the Sonny Bono act extended copyright length to the same as what had been harmonised in the EU a few years prior; Germany had had notoriously long copyright lengths, lasting 80 years past the death of the author at one point.
Don't get me wrong, copyright length is certainly too long, but blaming Disney is rather Americentric, considering the US was rather late to the game on long copyright durations.
For the CTEA (which is the one called the Mickey Mouse Act), that was only a few years after the EU copyright harmonisation in '93. Disney lobbied heavily for that, of course, and it was in their interest to do so, but it's likely it would have passed anyway if the EU had tied favourable trade deal conditions to harmonising copyright duration; the Senate report for the act specifically cites trade with the EU as a motivation.
The Copyright Act of 1976 increased the duration specifically in order to comply with the Berne Convention the US would end up joining. There's not a particular reason for why '76, other than legislators finally decided the US would join what was considered the international standard.
Both of these were more or less coincidences, in my opinion.
Don't get me wrong, copyright length is certainly too long, but blaming Disney is rather Americentric, considering the US was rather late to the game on long copyright durations.