It wasn't the "weapon of choice." Yes, the 18th and 19th century did witness lance-armed units like uhlans. However, lance-armed units were almost always a minority in post-17th-century European armies. True, there were lancers at Waterloo, but the majority of cavalrymen there carried sabres, carbines, and pistols. Indeed, most "lancers" didn't even carry lances. In many European lancer units, only a third of soldiers carried lances (the front rank), the rest carried sabres. A man couldn't use a lance very effectively if he was surrounded on all sides by his comrades.

Why the switch to sabres? It's difficult to learn to use a lance effectively. If you're not highly-trained, it is exceptionally difficult to use in close quarters. Also, lances used in the 16th and 17th century tended to be geared toward punching through plate armor (lance point carried by a charging man is very good at this). As guns made plate armor more and more obsolete, the appeal of lances waned somewhat. Sabres thus proved to be a good deal more flexible and useful than lances.