Cultural factors, as well as figuring out the best way to actually ride a horse. More can always be said, but as chariots come up every so often, I refer you to these previous answers:

• u/JoshoBrouwers has an overview of chariot combat in general, which also deals with their fall at the end;
• and u/PM_ME_UR_SADDLEBREDS looks at the reasons for the rise and fall of the chariot.

(Note that stirrups have absolutely nothing to do with this debate. Effective cavalry predates the stirrup. Alexander and his Companions charged home just fine without stirrups. So did cavalry in the Roman period, both Roman and their opponents. Anyone who says anything about stirrups is running with an old theory discredited not long after it was postulated, and the scholar who advanced that theory also clearly had never, ever, been on a horse, ever.)