Because I hate women characters who look like Ulti
They just look generic to me...
Black Maria on the other hand... the drawing of her eyes is very unique... her hairstyle is cool with TWO SWORDS used as her hair clipper...
Also, I like a woman with a naughty and MILFY attitude... they just... I don't know... my type LOL... How did I know she's somewhat naughty? because she sticks out her tongue outside...
Lastly, I'm NOT a fan of smoking smell... but she looks generally cool with that pipe... she's probably the second character in this series (which means SO RARE) to have a smoking pipe along with Madam Sharley... and I like madam sharley
Ulti just isn't my taste... she seems VERY straight forward... which translates to her words about Kaido.. I do not like women who are straight forward... I like them to be backstabing

Real BITCHES LOL
No but joking aside... I really like them to have some kind of cunning side... some hidden motives... some kind of mystery surrounds them instead of being too straight forward....
I mean... it's just the same when it comes to real life with people who date...
Having too much dandy character... always straight forward... it's just god damn boring...
On the other hand.. I'd prefer someone who can surprise me from time to another... this also happens with some ladies about their choice of their man.. which is why you see the whole "bad boy" being somewhat a sort of a highlight..
But for me... it's the same for women...
Like take Sakura/Hinata/Tsunade comparison for example...
Sakura and Hinata both are appropriate for my age... but I would choose Tsunade over them 24/7 (cause she is fucking fun)
I feem the same regarding the twisted/naughty/mysterious vibe I get from "Black Maria" compared "Ulti"
So... overall.. it's about both Design PLUS Character and vibe about their persona! All played a role for my choice