I think it might be useful in certain fringe campaigns, and probably many players would appreciate an occasional mass battle (especially if they are LotR movies fans) but the problem is that it is very difficult to design such system, WotC tried a couple of times in UA but scrapped both eventually.
The players would expect that their actions as individuals can determine the outcomes of the whole battle, but obviously they can't win the battle alone... so the system would need to represent combat differently (also for practical reasons, as the players will expect to be able to kill 100 orcs but without spending 100 rounds of combat to do so) but still allow the PCs to fight inside the mass. Having one-on-one duels against important NPCs is not a "mass" battle, it's a regular D&D combat encounter with two armies playing the role of background scenery.
In addition, trying to create a system where the PCs lead the armies (or specific units) changes the game too much towards a wargame, which is not what most players want. And if they really want it, it's easier to just pick a good existing wargame ruleset and play that instead, like the "war machine" from older D&D.
So to really be able to bring D&D characters into a mass battle is indeed a huge task. The final (assuming successful) result might as well be no more a couple of pages of rules, but to distill everything down to those 2 pages could take ages even for very good game designers.