He did attempt to explain why. Wikipedia: "On 4 February 1600, Kepler met Tycho Brahe....Tycho guarded his data closely, but was impressed by Kepler's theoretical ideas and soon allowed him more access. Kepler planned to test his theory from Mysterium Cosmographicum based on the Mars data, but he estimated that the work would take up to two years (since he was not allowed to simply copy the data for his own use)."

Mixed it up! I meant Tycho Brahe actually.

Sure he was. And then Newton came along and said it's all because of gravity and Kepler's laws were nothing but his laws of motion applied to planets.

Newton was a bit of a brat but everybody accepted his explanation. Then the problem turned to trying to explain gravity.

Thus science advances, one explanation at a time.

He might not have been able to explain why _but_ I'd bet anything he would have wanted to if he could.