Our government can't keep our basic infrastructure (ex. the northeast corridor or even the interstate highway system- witness the MSP bridge collapse) even in a "state of good repair". Given this, how do you expect them to design, engineer, procure land for, build, and especially properly maintain something to FRA 200mph standards?
The FRA's standards are pretty strict. The rules are pretty clear about how when the standards for a particular track class aren't met, you have to run the trains slower (though I think there are a few cases where you have a month or two in which to meet the standards before the trains have to fully slow down). If a lot of track that was supposed to be 200 MPH isn't, there's a good chance that the trains will slow down for a little while, the news media will notice, and the politicians will decide to throw money at the problem.
Sometimes building new things from scratch for $X is also politically easier than spending one twentieth of $X fixing something.
And I have to wonder if the sense of satisfaction engineers get varies. Would you rather spend a year of your working life building a new 200 MPH track, or doing a bunch of things along the lines of fixing some track that has a speed restriction of 90 MPH so that it can operate at 135 MPH like it's supposed to (and when you're done, still not being able to do anything about that catenary wire that won't let the trains run at 150 MPH)? (I assume the old catenary is not considered to be not in good repair merely on account of not being constant tension, but I don't really know.)
The US Navy's submarine force found that when the officers who had done best in school got to pick their assignments, the ships that were in overhaul ended up getting the weaker officers pretty consistently. The Navy eventually noticed that having weaker officers on the ships that were getting overhauled was a bad thing, and changed how they handled assignments.
If indeed the civilian engineers working on the railroads are going to prefer working on the 200 MPH track, there is no way in our democracy to force the best engineers to work on the stupid annoying little problems on the NEC. (And I strongly suspect that good engineers / management can do a lot to stretch budget dollars in a safe manner, where weaker engineers / management will take a less efficient approach and get less done with the same money.)
Notice also how the way Amtrak most recently managed to buy new rolling stock at all was that it was ``high speed''. Nevermind that the top speed south of New York City is a whole 8% faster for the Acela than the Regional, and an 8% speed difference probably barely has any effect on how many people are willing to ride the train. (It's more complicated than that, of course, since the Acela doesn't even get to enjoy that 8% speed advantage for the whole route, but the Acela also skips a bunch of stops, and can probably accelerate faster. But Amtrak could have simply bought some more electric locomotives and made a habit of running them in pairs and rewritten the schedule if they wanted the Amfleet I coaches to enjoy those last two advantages.)
I'm sort of curious about the NEC state of good repair issue. I do think I recall reading about it being an issue in the bill that Congress has almost managed to pass. But when I've ridden the NEC, I haven't noticed it being broken, and I suspect that the NEC in its present state is a safer way to travel than the current Interstate highway system, or any improved version of the highway system we might see in the next few decades.
I'm also wondering if the highway bridge standards are as clear cut as the FRA track standards. For some reason, I suspect they may not be, but I'm not really sure. I think I've seen news articles saying that engineers inspected some particular highway bridge and decided it was safe enough in some case in the distant past. I'm wondering how much subjectivity gets involved there; I'm reminded of what Richard Feynman discovered about various estimates of the safety of the Challenger. (Various lower level people at NASA apparently had a pretty good idea of the actual safety. Management figured they couldn't possibly be sending actual living human beings into outer space in something that dangerous, because if it was that dangerous you wouldn't want to send actual living human beings, so obviously it was safer than the people who were working for them thought it was. Unfortunately, we discovered that management writing wishful numbers down on a piece of paper didn't make reality so.)
While I do think we need to be working to fix the highway bridges (as well as thinking about whether we could save money in the long run in meeting our total transportation needs by getting some of the load off the highway and onto the rails), a single highway bridge collapse really doesn't bother me much. I'm too lazy to go find the real numbers at the moment, but if there were 150 people who died in that collapse and about 150 million Interstate highway bridge users in the US (counting both drivers and passengers), it would seem that collapse only killed one out of every 1 million Interstate highway users. There are probably a bunch of other ways of dying on an Interstate highway that are a lot more likely.