"Lancer" implies a mounted lance-wielding cavalry. D&D, and most rpgs, have not traditionally supported 'mounted' archetypes very well. It is hard to take one into the Dungeon after all. The table top war game genre, whence D&D sprung from, has all different kinds of mounted lancer types though. D&D has also not traditionally supported different weapon styles with much specificity at all over the years. Certain traditional medieval/fantasy archetypes certainly call to mind specific weapons: dwarves with axes (mainly due to Gimli & tLotR), rogues and daggers (though this mainly due to restricting weapons by class and class features such as sneak attack, 4e was the only edition that really incentivised daggers for Rogues), virtually any fantasy & swords. D&D did seem to popularize the cleric hammer/mace meme with the 'no edged weapons' thing, relying on Bishop Turpin legends for inspiration. The spear has not really been prominent in any of these archetypes, outside the mounted 'Knight' type (which D&D does not do well), and phalanx type formations (Greek Hoplites, Roman legions with javelins, pike lines), which D&D also does not do well (but table top war games do very well). All this despite the fact that the spear was probably the most historically prominent and ubiquitous weapon of antiquity. Perhaps it was viewed as to 'mundane' for the legends and myth to grow around it as grew up around swords.
In short, the prominent fighting weapons of D&D are usually that way due to: a) class restrictions (cleric, rogue), b) mythical associations (numerous swords of legend), and c) being the mechanically 'best option' or most versatile weapon available. Spears have never quite had the stats to make them that desirable, despite numerous reach rules implementations over the years (possibly because these rules were not always implemented consistently across groups). I guess you can't just clip a spear to your belt/back and forget about it until you need it again. Furthermore, I would add that D&D combat mechanics have always been somewhat abstract and has not, IMHO, been well served by attempts to make specific weapon tactics distinct in more than the most general sense, even through feats (UA or elsewhere). If one wants a 'spear-warrior' one aught to just give the spear slightly better stats, and people will play spear warriors.