Because other missions have no where near the density of loot, at least not on anything that is 100% assured to spawn, sometimes many times per month. There was a time they where the single most lucrative encounter in the whole mod. Probably not the case anymore, but still pretty high.
Basically about half a base worth of loot with significantly fewer lower ranked defenders and only guass, and not full plasma, blaster launchers, and terror units. Something else was needed to give some more risk. First it was making the ship blowup its contents on a timer thus depriving you of loot unless you played more risky. Still wasn't sufficient but cutting below 20 turns wasn't feasible. So give the baddies more bite with a high impact but not necessarily always effective weapon.
So get a high risk, high reward scenario, that is not easily farmed at least until the very late game at which point the loot/income impact is no longer a balance issue. It also serves as way to introduce the home grown earthy version of the blaster bomb, in a way that get players interested.
