For the old and grizzled World of Warcraft vets, dungeon delves were a longer process than they are nowadays. Why was that, exactly? A video from YouTuber Hiru takes a closer look at this question and provides some pretty succinct explanations.
In short, WoW dungeons were the size they were because the developers assumed everyone would only run them one time. The paradigm of MMORPG dungeon design at the time of WoW’s development was EverQuest, which featured non-instanced dungeons that were sized in such a way that they could accommodate everyone on the server. A number of the WoW devs were hardcore EQ1 players and so they decided to follow the example.
Of course, the problem with vanilla WoW dungeons being the size they were was it ran counter to the game’s gearing system, which pretty much required players to run these instances more than once to get the equipment they wanted. So, dungeons were shortened in the next expansion and onward both in response to this design decision and player feedback.