There were several reasons.
- Good tactics. Polish hussars' charge was much more than a bunch of riders galloping at you. Usually hussars started their attack in relatively loose formation, which made them less vulnerable to musket fire. Short before clashing with the enemy they would close their ranks creating pretty much a wall of steel and lances. If the first attack didn't succeed, they would withdraw and immediately hit you again, and again. Usually however, the first strike was enough. Hussar units were also quite fast and maneuverable.
- Good equipment. Hussars started as rather ligth cavalry, attacking in close formation with lances, with shield as their main defence. With time they started wearing steel helmets and chainmails, later plate armour. The armour mostly consisted of breastplate, backplate and protection for neck, arms, often also forarms. The armour was relatively light, which allowed the hussars to move fast, but offered surprisingly good protection, even against musket fire in 17th century.
- Good horses. Actually, excellent horses, very strong, fast and intelligent and very resilient. Hussars's horses were very carefully selected and trained, they were extremely expensive. Hussars always tried to give them the best care possible and often considered them comrades in arms, not simply animals. Thanks to those horses hussars were deadly quick on the battlefield.
- Good weapons. Polish hussars' main weapon was a "tree", i.e. very long lance; they were hollowed inside, which made them lighter, they also would break in the first clash, but usually it was enough. Against lighter armed opponent (like Tatars) hussars often used lighter and shorter lances; in close combat hussars used estocs and then sabres, they also were armed with pistols. That was the companion (towarzysz) a knight in the hussars' units; each companion had also retainers, with less expensive arrmour, armed with sabre, sometimes a light lance, and arquebuse, who fought in second and third rank. In short Polish hussars were perfectly capable of dealing with any kind of opposition, from pikemen to enemy cavalry.
- Good training and high morale. The service in hussar unit was very prestigious but expensive (soldiers had to provide their equipment on their own, except the lances) so only those who really, really wanted to be hussars were able to join them; high morale made them also work hard while training, since "noblesse oblige".
- Psychological effect. There is still debate concerning wings - were they used in battle or not (some sources claim that they were); however even without wings a Polish hussar in armour covered with tiger/leopard skin made an impressive figure. And a wall of steel bristled with lances coming at you with amazing speed is not something you want to see.
In short Polish-Lihuanian hussars were quick, well armoured and armed, very well trained and with high morale. They were more or less an equivalent of tank units which, correctly used, were indeed almost unstoppable. They used experience from wars against western style units (like Swedish) and eastern style units (Ottomans, Tatars). They weren't always victorious but until the end of the 17th century they were a terrifying adversary