The lance has two main advantages over the sword: better reach, and better performance against armour. It has two disadvantages: it is more easily lost or broken when used, and is less wieldy if fighting in-close.

By the 18th century, armour was largely gone from the battlefield, made obsolete by guns. Anti-armour performance no longer mattered as much when choosing between lance and sword. As for reach, from about 1600, the pistol or carbine had largely replaced the lance as the main weapon for cavalry, provided both more reach than a lance and even better performance against armour. With cavalry often carrying a pair of muzzle-loading pistols, only allowing for two shots before reloading, a sword was an important sidearm. Thus, the key cavalry weapons shifted from lance (with a sword (or mace, or axe) as a sidearm in case the lance is lost) to pistol/carbine and sword.

The lance remained in use, since the lance still offered a reach advantage over the sword, and could be useful against cavalry, and also offered a reach advantage over the bayonet. The low point in lance usage was between the pistol taking over from it as the main weapon for cavalry, until the bayonet replaced the pike. The pike provided more reach than a lance, but the replacement of the pike by the bayonet made the lance more useful again.

More on lance, pistol, and sword in https://www.reddit.com/r/AskHistorians/comments/ekcozp/considering_how_effective_the_lance_was_at_the/

It requires much more extensive training and skill to use a lance, than to use a sword. A mediocre horseman and fencer can still bash away and be marginally useful. The same man with a lance, is not only useless, but also a danger to himself and his comrades. By basing the entire cavalry on swords, you increase the number of useful recruits and decrease the training time.

Also, there was no tactical necessity for lances during the 18th century. Cavalry was expected to fight against other cavalry, not against steadfast squares of infantry. In a cavalry melee, swords are the superior choice. Cavalry would only attack infantry if they had good reasons to expect the infantry to rout at the sight of cavalry charging at them, or if it could be caught in the flank or the rear, or if the infantry was already on the run. You don't need a lance for that job. Cavalry swords are quite long, and if the infantryman can't or won't point his bayonet at you, you don't need the extra reach that a lance provides.

This of course changed in the 19th century. Infantry not only multiplied in numbers, but also deployed in more dense formations. Changes in doctrines also enabled a quicker transition to square formation. Whereas the Austrians and Prussians only fielded uhlans because they had large Polish populations (for some reason, the tradition of lance wielding was kept alive in Poland while it died out in the rest of Europe), the French deliberately trained lancers to counteract the new challenges.

Still, you must keep in mind that, even in dedicated lancer regiments, often only about 1/3 to 1/2 of the troopers carried lances. It was thought that, while lances can be useful in the front rank, they make little sense in the second and third ranks.