I see the pylons being the result of megastructures being used in other ways around the galaxy, leading to large scale manufacturing being needed.
Maybe a starbase with walkways extended between parts of a planetoid mass, which needed single piece metal supports of some impressive size and tensile strength. Underground mining facilities from Devil in the Dark, Memory Alpha, early shipyards etc
Making smaller ones for starships to create thin cavaties in the profile for warp flight would seem easy compared to that.
Allowing something as heavy as the nacelles to be carefully carried away from the hull by only the bare necessary mechanical connections.
Starbases and starships have been shown to be similar in that era, there are bound to be a lot of shared manufacturing techniques going on there, saving time and resources by maximizing shared production.