Loose rivets,
‘I assume there must be a good reason it's hinged at the rear.’
A logical view, but the reason probably lost in the 707 records, and a situation where today’s 737 engineers wish it wasn’t so.
Re; tail area increase.
From an aerodynamic view, the increased area was required for the longer, heavier aircraft, more thrust, etc. The forces might be higher, but the overall balance of forces was maintained.
From an engineering view, if the balance was disturbed with significant trim displacement, then the individual higher forces on components could be a problem (hinge moments).
The classic 737 had a problem with trim runaway; it was difficult to trim the horizontal stab with opposing elevator applied. The force imbalance was resolved by unloading the combined ‘stab - elevator’ with a roller coaster manoeuvre, the trim wheel being moved when the elevator was inline with the tail - reduced elevator reaction. This was the basis of the original certification and the forces required to balance the smaller aircraft.
The larger aircraft have increased stab area, the forces are still balanced, but individually could be higher. The need for the trim runaway manoeuvre dipped below the training horizon, possible with a self satisfied ‘it hasn’t happened yet’ (but still the same risk).
But the risk might not be the same.
The trim runaway drill requires pilot recognition and intervention to electrically isolate the trim, and then recover normal trimmed flight using the trim wheel. If the forces involved are higher than the classic, then more nose up pitching moment might not be available due to revised aerodynamic, mechanical, or physical limits, i.e. unable to overcome the higher stab nose-down pitching moment, elevator hydraulics suffer jack stall, or pilot strength. If so then the trim runaway manoeuvre might not be practical.
The crew’s involvement could reduce this problem by holding the trim wheel and with the speed of isolating the trim before reaching the limit condition - not necessarily the stab limit, but the point where the elevator is ineffective. Thus the basis of certification did not change, but the assumptions within the trim runaway drill did. Apparently these were not overtly recognised, but recall the EASA query about the full range of trim in the NG, this also introduced speed; higher speeds higher forces.
The Max has different aerodynamics / thrust, etc, (need for MCAS), so again whilst there was a change in force, but the balance was maintained. This could imply that the trim runaway recovery limit had less margin in crew intervention - time for recognition and action - ineffective elevator for less trim deviation.
The lack of factual evidence from incidents in the NG does not identify any change from the classic runaway drill, but the simulator training tests do (video).
Regrettably, evidence from the Max accidents appears to confirm these difficulties, which could be greater than in the NG (note Max simulator inaccuracies), particularly the effect of speed.
If the suppositions above have value, then the trim runaway drill in the Max requires urgent crew intervention - recognition and isolation, and simultaneous control of speed, and possibly altitude constraint.
Thus the revisited certification question is if the crew participation is realistic for a trim runaway in the Max; a change from the NG.
With AoA input to MCAS clearly it was not, but MCAS will be modified eliminating that specific failure; but the trim runaway condition remains.
1: Yes the human will manage with yet more training, recognise and act quickly. (Currency, memory recall, surprise, experience)
or
2: The accidents identify the realistic expectation of crew intervention for trim runway - time to recognise, time to act, startle, range of piloting experience. (Irrespective of MCAS / AoA distractions)
A judgement; but who judges ?