I used to watch a lot of professional starcraft 2 casts back in the WoL and HotS days, and usage of mothership and carrier was extremely rare, to the point where casters would get very excited whenever a fleet beacon was laid down.
After a 5 year hiatus, I've started watching a few LotV casts, and the commentators talk about the standard carrier/mothership combinations in Protoss late game. They appear to now be quite powerful, which puzzles me because they appear to have the same stats as before.
I am not about to go through every patch note for the last five years to hunt down this change, because I understand that even subtle things can change a unit's usefulness drastically. Things like build time, acceleration speed or even changes in the game economy that allow more units to be mass produced. If I were to guess why carriers (and battlecruisers, for that matter) are now popular, I would say it's a change in the game economy (More bases are available per player, and the maps are larger and contain more resources) and the presence of teleport abilities that allow these otherwise slow units to have a speedy retreat. But I do not actively play SC2 multiplayer so I wouldn't know.
What has changed between HotS and LotV to make protoss carriers and mothership so strong and commonly used?