It’s because of behind the scenes drama. The original show runner wanted everything to be more distinct. He wanted the Klingons to be a lot more alien, and told the Klingon ship designers to ignore all previous Klingon designs.
For the Federation ships, he mandated no round nacelles, and to use flat profiles. We have no idea why, it’s just what he requested of the designers.
It was too late in production when he left for them to re-design everything so they just kept them, and started somewhat fixing things with season 2 instead. Which is why we got a D7 that actually looks like a D7
Was that show runner Bryan Fuller then?
Weird, I wonder what his issues where? I never understand - and the goes back to both Enterprise and the first 2 Kelvinverse movies - when somebody says "I want to make a new Star Trek...but I don't want it to be too...you know...Star Trek-y."
Fine, then make something else!