According to what PeteEU said, a 225 mile trip on French high speed rail only takes about 20 minutes longer than a 225 mile trip on Acela. If there were an hour difference, I might believe that the reason it's not working here is just because of speed, but when the difference is that small, there's something else behind the fact that it's not profitable.
Acela is not a high speed train system per say. The problem with Acela is it utilizes existing tracks for the most part and does not have exclusive access on the tracks... aka higher priority trains can stop the train. The average speed on the trip is 86 miles an hour and that is pathetic frankly. This includes the slowing into stations and out of stations..
The average speed on the French system is 173 miles an hour, the Japanese 125 miles an hour and the German system that runs an average of 153 miles an hour. This is considerably higher speeds on average.
To be a real high speed rail system, it needs to be on new tracks that are designed for it and the tracks need to be as straight as possible. Acela route and tracks do not meet these requirements and hence the train might be fast on a test track, but in reality it is a "high speed train".
As for its profitability. Profitability is directly in my opinion directly linked to destinations, speed and in some part price.
If I can get from Malaga to Madrid in 2 hours and 40 min (city center to city center) for 45 euros (50ish dollars), then that is far far cheaper, faster and far more comfortable than taking a plane.
But lets look at the French system. Trip is one way.
Paris to Lyon (the first high speed in France) is 264 miles. It takes 1 hour 56 min and between 58 and 79 dollars for a standard seat.
Paris to Marseille is 366 miles (the longest route). It takes 3 hours and costs 75 to 89 dollars for a standard seat.
Now a plane ride to the same destinations.. Note these prices are roundtrip, since one way fares have insane penalties price wise.
Paris to Lyon. Takes 1 hour 10 min flying time and costs from 75 to 100+ dollars on Air France, but you have to add 68 dollars in taxes and fees. On top of the flying time, you need to add check in, driving to the airport in Paris and driving from the airport in Lyon.
Paris to Marseille. Takes 2 hours 15 min of flying time and costs again from 75 dollars to 100+ dollars on Air France for the cheapest. And again you need to add 68 dollars for various taxes and fees. Again you need to factor in driving time to and from airports, and check in/out and so on.
Now I suspect you can get cheaper fares from other airlines but I could not be bothered to find those out. However what you can not avoid is the 34 dollars taxes and fees each way regardless of what airline you go with. So if an airline need to beat the high speed train, then they basiclly have to sell tickets for 16 to 25 dollars which I guess is always possible. But considering that the train fares are one way, and to even get close to the train fares you need to buy a round trip ticket, then well...