Because they're basically support ships for their own little team. Also because Cryptic still cared about balance back when it designed the original carrier, the Vo'quv, and its close cousin the Atrox. They were worried that a tactical-heavy carrier would be too powerful.
And now we've got the JH Dreadnought, in which you can sleep your way to 15k. But there wasn't anywhere else to go, as sci-carriers were pretty saturated.
Because they are what the Klingons do for 'Science' and 'exploration'. Their 'Science' is the science of taking entire systems with one ship(For a certain definition of 'one ship'). Outfitted for the longest trips, with the heaviest hulls, and a mess of support vessels carried, these ships are often charting the unknown and bringing it into the Empire.
Hells, the ship is the Klingon Dreadnought in NPC terms. It deserves more credit.
Why do you think my Gorn engineer flies a MIRROR carrier?
I use the MU carrier too! Mainly because it was a lot cheaper than the Prime one. Got it for 50k EC instead of the 200k Dil pricetag of the other one.
Standard Tier 5 ships are waaaay to expensive at 200k Dil. I remember when they were 126k EC back in the day. You could decommission and rebuy at your pleasure w/o having to clog up your 'space-garage' or buy extra ship berths.