Newton and Einstein answered "how" questions with more generality and more precision.

Kepler's model predicted the behavior of the planets, and so does Newton's model, but Newton's model is more general, applying not only to planets, but also to apples, cannonballs, trains, and other objects with mass.

Einstein's model is more accurate and precise than Newton's model, especially about the orbit of Mercury. Einstein's model also predicts gravitational lensing (e.g. during eclipses), which Newton's model doesn't.

"Why" questions are very hard for physicists. Generally, when we ask "why" questions, we're asking what prior Event X caused Event Y. But for universal laws, there is no prior event that caused the universal law to be true. The best we get is more generality. Planets orbit the sun for the same reason apples fall to the earth, but we don't know why gravity happens, only how. Magnets resist each other for the same reason the floor is hard, but we don't know why magnetism happens, only how.

Newtonian mechanics doesn't explain the why of itself, it explains why Kepler's laws are true, an explanation that won't make sense if you don't know what Kepler's laws are saying, and which won't hold any significance to you if you don't know why Kepler's laws are thought to be true.

Weren't Kepler's models worse at prediction, at first? We adopted them because they were more parsimonious.