Initially, because net neutrality was just how business was done. It also helped that while there were certainly commercial applications of the early internet, they were fewer, and business-to-business more than business-to-consumer. Consumer applications of the internet were essentially non-existent outside, perhaps, informational and communication oriented, not consumption oriented (in the media sense). Then by the 90s you were dealing with more, and local, ISPs. These ensured a certain level of cooperation or you'd risk being booted out because you'd be known as a bad actor. This is also when consumer applications for the internet really start, but were also generally lower bandwidth (certainly due to the last-mile connections being slow, 28.8k to 56k modems, at the time, ISPs could handle you maxing out your 28.8k modem because you could only do so much damage to their total bandwidth pool).
Then with broadband you end up with users given access to a larger portion of the pool, akin to overbooking an airline. If everyone in my town gets 1Mbps but the local ISP can only handle 1Gbps, well, 1000 users maxing it out will shut the rest of the town out. This gets to a legitimate case for traffic shaping, ensuring equal access to the common resource pool. Those users can still max it, but only when everyone else isn't using it (or using it minimally).
However, traffic shaping (and related things like blatant packet dropping) started being more generally applied, to whole protocols (bittorrent) or services (Netflix, et al.). The latter example is a particularly good example of a conflict of interest. Companies like Comcast control so many user experiences in the US, but they own NBC and other media brands, as well as providing video-on-demand services as part of their cable offerings. They have a clear reason to want to reduce the QoS of Netflix and can, legally, without network neutrality. This is directly harmful to customers experiences, and a bordering on an abuse of market position.
===
Really, though, it's silly to look at how things worked 30 years ago and think the exact same rules will apply today. The circumstances have all changed. Broadband-requiring, consumer-oriented services didn't exist 30 years ago. They do today. ISPs today are screwing over consumers and prospective new businesses. They are the robber barons of the digital age, imposing their taxes at each bend in the river and screwing over everyone upstream and downstream to make a buck.