I guess for trains it's quite similar than with cars: After a certain point of speed, consumption goes up so much it's simply not worth it.

I can drive my car at 80km/h or 120km/h for about the same consumption (5l/100km) - when I go up to 150 or 160km/h, I'll have an increase of about 20-40%, still reasonable for the speed I gain (compared to 120km/h) - I gain about as much time as the extra consumption is.

But if I press it above that, consumption does increase much more than the raw speed gain. I can imagine that 300km/h for a train is about the same as 150km/h for my car. Go above it, and it uses much more electric power than going just a little bit slower.

And considering the HSR operator probably got told to be more efficent, they'll keep it that way, unless going at higher speed gets them more money.