And he we go again. Lets make a list because I like making lists.
Main characters who get significant amounts of development and backstory after two seasons (29 episodes plus a couple shorts):
Burnham
Lorca
Stamets
Culber
Tilly
Saru
Tyler/Voq
Pike
Georgiou
Recurring characters who receive development and backstory:
Cornwell
L'Rell
Sarek
Amanda
Reno
Leland
Spock
Poor Airiam
Mudd
Number One
One-off characters who receive more backstory than Morn:
Gabrielle
Siranna
Poe
Tenavik
So, not even counting the one-offs, we have 19 characters who are more fleshed out than Morn, the literal prop for a running gag, whose bits of backstory we do get comes mostly from an episode where he's just providing the McGuffin inciting Quark's story. By the way, how much do we know about Morn after 29 episodes of DS9? Because the bulk we know about him stems from said season 6 episode.
But I get it. Previous Star Trek stories were a bit formulaic, weren't they? You fill out your Bridge crew/command positions and there, your cast. No need to figure out if the character and position they are in actually are that compelling, or lend themselves to tell good stories about them (like the oft maligned Mayweather).
But here? There's characters on the bridge, and they are not in the focus of the story?! Clearly the story sucks at developing characters because the people who would be the focus by custom in other shows aren't in this instance. How?!
All snark aside, there's plenty of characters developed way more than Morn, but somehow they don't count because they don't fill the specific slots main characters used to fill in the other shows.
And by the way, having visually distinct, named background characters isn't a weakness. It provides visual continuity and having a reservoir of pre-existing characters to pull from if you need them is good, because you can pull an O'Brien, who if you recall started as a named recurring face in the background.