The issue is always space: the utility cruser is going to need space for weapons, space for cargo, space for Engineering Stuff, probably space for a medical bay and modest lab...
This problem is only made worse by the increase in interior volume consumed by the more advanced technology- Phasers and the Warp Seven core are double the size of the warp five cores and phase cannons, the deflector has already gone from one deck to Three and I don't expect this to get better anytime soon. (Staring at Phaser Strips being like an entire deck in horror* but yeah
We build a lot of saucers because of how they're the option that gives us the most space, which is, you know, really important for anything expecting to do more than one thing with any great competency.
You might note that it's Trek canon that ships that do only one thing well, tend to get memed on by ships that do lots of stuff good enough, usually with a Starfleet logo on the side.
"Jack of all trades, master of none, but far better than a master of one" is in full effect.
And while it's later down the timeline, I really can't think of any better example than the D'Deridex.
Which is a ship newer than the Galaxy class and a pure WarShip explicitly designed to counter the Galaxy class...
Which can only defeat a Galaxy with 2-1 odds and surprise, 3-1 odds and luck, or 4-1 odds and neither... While being about two thirds the cost of a Galaxy and so in no way able to actually achive those ratios reasonably.
Specialist ships just don't do very well, in the Trekverse.
