A lot of that had to do with limitations they had. Heroes and hero units were their way of circumventing some of those issues. When SC1 was made, they didn't have the time and luxury to make extra special units for singleplayer when they were already focusing so much on multiplayer. They created units like the Valkyrie in SC1, and later released it for Brood War. They would have had to find time to polish up that unit if they were to use it in SC1 singleplayer, which is time they didn't really have. That's why we see Valks and Dark Templar in Brood War.

War3 changed this. They had time on this project to focus on both muiltiplayer and singleplayer. There are lots of units you could get in War3's singleplayer that you couldn't otherwise get in multi.

SC2 continues this trend by adding a lot of units into the singleplayer because they can do it. This is something I think they always wanted, but never had the time or resources to achieve.