As the person who implemented it originally, it was not a technical limitation but a design choice.
In the early implementation of Warcraft 1 (1993-ish) I made it possible to select all the units on-screen at once with drag-select, and even more by scrolling and shift-clicking or shift-click-dragging.
Allen Adham (president of Blizzard, and exec producer for Warcraft) argued convincingly that only 4 units (for War 1) should be selected at a time. I argued against it pretty vociferously at the time … and in later days (post launch, most likely) came around to his way of thinking. Attacking with a superfluid of units takes less skill than selecting troops in small batches, and so requires to use more intent & skill.
Warcraft 2 allowed 9 units to be selected, and StarCraft more.
Thanks. Much of the discussion I used to enjoy about post-mortems in game dev. There was some kind of choice that went into the design process.
On the unit selection, a personal view on the situation is that nigh-instantaneous unit commands and obedience approaches valley of the dolls. It becomes too much like Wargames calculating theoretical death scenarios, abstracted from the issues of actual army logistics.
It takes some non-zero time. Units are not always doing exactly what they're supposed to. The fog of war is often worse not better than the game abstraction.
I don't think it's about blizzard but gaming culture in general that caused the gradual change. Gradually lowering the bar on skills required to play in some way.
Starcraft 2's F2 to select the whole army on the map is the other extreme and where it eventually landed right (I think that was introduced in a SC2 expansion, it was not a launch feature?)
I personally always hated limits on selecting in RTS games. Anything that would require micro-management was a turn off for me: it was immersion breaking, like that tank just sits there doing some stupid thing because the high supreme commander hasn't personally reached out to him to give a specific order what to do in this specific combat engagement. No.
In the early implementation of Warcraft 1 (1993-ish) I made it possible to select all the units on-screen at once with drag-select, and even more by scrolling and shift-clicking or shift-click-dragging.
Allen Adham (president of Blizzard, and exec producer for Warcraft) argued convincingly that only 4 units (for War 1) should be selected at a time. I argued against it pretty vociferously at the time … and in later days (post launch, most likely) came around to his way of thinking. Attacking with a superfluid of units takes less skill than selecting troops in small batches, and so requires to use more intent & skill.
Warcraft 2 allowed 9 units to be selected, and StarCraft more.