I just thought of another (theoretical) explanation for getting down on one knee when proposing marriage.
In cinematography, when you're looking down on your subject, it means they're weak, helpless. Conversely, when you're looking up at your subject, they're powerful.
Most men are taller than women right? That means most of the time, they're more powerful than women, if we use the idea from cinema.
Now when men propose, getting down on one knee makes them look weaker, and women, stronger. Therefore, it makes the decision easier for the girl because she feels empowered.
Is this a load of poo or does it make sense?