The use of the STO assets was purely a budgetary reason. And a correct one, in my opinion. Generic far background ships don’t have to be as detailed or well-designed, because they’re not meant to be seen up close; they’re just padding. It’s no different from when DS9’s later seasons used model kits as far background ships because they no longer had access to the original filming models.
Indeed, STO's newer models are approaching mid-to-late '90s TV CG standards. I suppose the heavily-optimized texturing would also make them a lot easier to convert to being screen-ready than the more-detailed Eaglemoss CG library.