Because Zoans are ass outside of the mythical ones.
You need to have a separate skillset to add on top, otherwise you're just a hairy jobber.
Imo, this is why Enies Lobby fights were pretty top notch. They used their devil fruits as an advantage with Rokushiki.
Problem with this also depends on Oda'a ability to create unique movesets. WsW had the exact capabilities as Lucci (or at least should have), yet the fight was a lot shorter and varied. The only difference was Haki and a few new Rokushiki moves that were shown off once or twice.
Page 1 unfortunately did nothing. As did Jack. Ulti at least fit the motif of her dinosaur fruit. Drake has done nothing outside of I assume gain speed/strength to use his mace/sword. Black Maria and Sasaki were certainly the most unique of the bunch, but still fell flat for me.
There IS however hope for King and Queen. Queen has shown a ridiculous amount of variety in his moveset, especially being a cyborg. King has no defined fighting style, and has shown his share of unique stuff too, without even going Hybrid yet. Imo, these 2 100% feel like Enies Lobby opponents and I have hope Oda does them well enough.