Since when were carriers and mothership so good?

Solution 1:

Short answer: Meta changes

Long answer: Games are accelerated these days, HotS and LotV differences were additional early game units and more starting probes. A typical opening for Protoss is to use Oracles for harassment, giving you a Stargate already. Since you've already tech'd into a Stargate, players started using it more often.

Immortal + zealot ground forces are usually enough to clear the ground, but players were needing something in the air. Void rays take too long to charge up, and tempests are a decent counter unit only so the choice was carriers.

The Mothership is useful because it cloaks the carriers and prevents Vipers/Mutas or Battlecruisers/Thors from killing them, giving them added safety for one unit.