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.
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.
Sometimes I think I play STO just to have stuff to rant about on the forums!
That wouldn't earn nearly as much $ for Cryptic erei1 and this game is all about grabbing money.
I would have prefered that carriers were limited to no higher than a Lt.Commander tactical, for me the Karfi is as tactical orientated as a carrier should go. I would like to see more engineer heavy carriers though with 4/4 weapons but only and ensign and/or lt tactical station. Though that would trully stick twos up at cruisers.