I believe there are several things at work here, and safety concerns are one of them. (@Ana--unless construction was egregiously bad, you wouldn't expect substandard materials and shoddy workmanship to show up immediately as crashes and derailments--it would take awhile under normal system operational use for internal concrete and metallurgical stresses to reach a breaking point and fail under load.)

I'm also of the opinion:
--The energy conservation of lower speed operations is definitely a driver and probably comes from the top brass in central govt.

--Attempts to ease disgruntlement by passengers at having their low cost slow train options curtailed, only to be forced onto high cost bullet or back to intercity buses. So yes, some "social harmony" and placation may be at work here. Frankly, I think that even 2nd class bullet ticket prices are quite high compared to ordinary Chinese incomes. The State is subsidizing rail service, and it appears they will have to subsidize it even more.

--Balancing behind-the-scenes grumping from airlines, who see some real dangers in competition from a fast rail service that can bring total journey times in close range to air service. (air = flight time + to/from airport time + check-in/security/boarding wait time).

I await with interest the start date of the BJ-SH service, the real-name registration system (I can't believe they will do this on the short haul milk run bullets like BJ-TJ or NJ-SH), and the online reservation system. Lots of simultaneous learning curves for everybody to get down.