I'm no expert but I'll take a stab at it. The faster you go the more drag you get on the fuselage that would need to be compensated for with more fuel (unless some neat mechanic helps to mitigate that). Take a look at a conventional jet airplane and you'll notice it's capable of holding passengers from nose cone to tail reasonably well due to its cylindrical shape. The X-59's design has some very interesting geometric features that would give less internal volume for passengers (unless it can be modified to improve for this).

If that's not reasonable enough then just look at the kinetic energy equation, KE=1/2mv^2. Compared to a velocity of a jet airliner going at 900km/hr versus this plane's Mach 1.4 (roughly 1500 km/hr) it takes roughly 2.78 times more energy to move a vehicle at that speed (not accounting for drag, energy efficiency, etc.). Is it worth spending roughly 2.78 times more fuel to get to a place 1.67 times faster?