Other than stating the mathematical relation which is the basis for all of quantum mechanics, I do not know WHY nature favors this way of doing business. The statement is that if you have two conjugate variables such as time and energy, or position and momentum, it is not possible to specify BOTH of them with infinite precision. The reason for this is usually explained by way of how we make measurements using reflected photons of light. If we want to specify the exact position, we need to use photons of vanishingly small wavelength. This means photons of very high energy since E = h c/wavelength. But if we wallop the electron with such a photon to precisely determine where it is right now, we loose control over exactly how much energy, and the specific momentum ( mass times speed in a well-defined direction in space). Epistemologically, I do not know why this should happen, other than it seems to be the way the world is designed.