Games in FAF end because (1) someone gives up (2) an ACU dies or (3) because the battlefield gets more and more dangerous over time.
Factor (1) is going to be based on (2) and (3) anyway (people give up because they would lose for other reasons) so it's not worth looking at.
Factor (2), ACU death - basically ACUs die due to snipes or due to overwhelming military force and they can't escape. Both of those are good mechanics that promote fun gameplay.
Factor (3), that the game gets more dangerous: what I mean is, at minute 4, you're not worried about a corsair snipe. At minute 10, you don't need to worry that a T3 heavy arty across the map is going to blow up your base unless you put in lots of shielding. As the game goes on, more and more dangerous stuff comes in, like nuke launchers, long-range artillery, the possibility of 20 strat bombers or an experimental showing up. And this makes it harder and harder, as the game goes on, to keep your stuff safe, and by stuff, I mean everything: your armies, your raiding units, your air force, your bases, your mexes, your ACU.
RAS SACUs mess with this because they provide mobile economy and are hard to kill. They are very survivable when everything else is in big trouble. They provide a kind of loophole from what is otherwise a consistent theme.
In StarCraft, the same thing doesn't really exist. In that game, the primary game-ending mechanics are either your base gets completely overrun, or you run out of minerals and you can't mine more. (Or: you realize that such a fate is inevitable, so you give up.) If someone added a RAS SACU unit to StarCraft, let's say there is a "Prince of Zerg" unit that can spawn new mineral patches, it would completely ruin the game. RAS SACUs are, in a small way, ruining FAF. So let's get rid of them.