what is main reason why this is very hard to achieve
Human powered flight generally? The most basic reason would be insufficient power-to-weight ratio.
In order to hold an airplaine aloft, you have to accelerate air downwards. For any given total weight of flying vehicle, the product of added vertical speed and volume of air per second which is given this acceleration kick is something you can not change. Only choice is either accelerating small amount of air to high speed or big amount to lower speed.
But energy/power you need to put into accelerating this airmass increases with square of added velocity (while provided lift depends on velocity only linearly). So it is always much more efficient to be pushing down big volume of air by only little (compare to induced drag of airfoil).
Theoretically, this energy necessary for staying aloft could be arbitrary low, jast take a sufficiently big amount of air and push it downwards only a little.
Well, not exactly. Important question is where are you getting the air to accelerate from. And here starts all practical problems.
You can either increase wingspan, so aircraft ploughs through more air at same airspeed, or you can increase airspeed. First comes with penalty in mass (so you need even more lift and many related issues), second results in increase in "common" parasitic drag -- power necessary increases with third power of speed -- so there is relatively hard celing (you can not expect being able to pedal big airplane much faster than your common bike regardless of any lift being generated or not, can you?).