Emissions, lighting, crash testing, and a bunch of other things would be needed for the EU Fiesta to be imported. It was crazy the amount of stuff that was different between the EU and US Fiestas and just about all of it due to regulations. Even the mirrors and wipers from the EU car weren't in accordance with US regulations. With the Focus RS, the platform as already US and EU compliant, so not nearly as many changes to bring the Focus RS here. The new EU Fiesta wasn't intended for US consumption, so bringing it here isn't an option.
I do agree that the ST line is becoming SRT. Ford is heavily targeting Jeep with their Bronco and some other new stuff. I always thought of FCA as the worst of the automakers when it comes to quality. They build some incredible machines, but not quality and reliable machines. I don't know why Ford is getting obsessed with chasing them. I still feel like Ford is trying to be too many different things to too many different people and different markets, all while ignoring many of their current markets. They want to be Waymo with autonomous. They want to be Tesla with electrics. They want to be Jeep with SUV's. They want to be all things truck. They want to be SRT with the ST products. The one thing they don't want to be is a car company.
In the end, ST will become another useless trim package like "sport", XLT, Titanium, or whatever trim name that has been applied over the last 30 years.