They also went to a 24-hour cancellation policy instead of the mooted one-hour policy, which is not an insignificant improvement. I'm just going to call this a hunch, but there was probably some negative feedback...but then again, Amtrak has trial ballooned this sort of thing at least once before and been forced to back off.
I'm still surprised there's not a move towards a "First Flex"/"Business Flex" fare (technically speaking a Coach/Acela Business Flexible fare is more flexible than what's available in Business/First), but I figure that's coming an iteration or two down the line.