Although Drilbur is a popular Pokemon in Pokemon GO due to its ability to evolve into the powerful Ground/Steel-type Pokemon Excadrill, it has yet to acquire its shiny form in the popular mobile game.
Although several Pokemon throughout Pokemon GO's roster possess their shiny form in-game, others have yet to receive the same treatment, and Drilbur currently finds itself in this group.
The details are not specific, as Niantic has stated in the past that it would be incredibly difficult to implement every shiny form within Pokemon GO, so the problem likely lies less with Drilbur specifically and more with the constraints that the mobile title possesses.
Pokemon GO: Will Drilbur ever receive its shiny form?
Although shiny Drilbur isn't currently available in Pokemon GO, shiny Pokemon are being introduced or tweaked quite often. For example, Druddigon was recently implemented into the game during the Dragonspiral Descent event and its shiny form was added at the same time. However, not every Pokemon is included with its shiny form, giving Niantic room to add the form later at their discretion.
Niantic doesn't typically announce when specific shiny forms will be included into Pokemon GO, so players are likely to be left playing the waiting game until the decision is made. However, it should be noted that Drilbur and Excadrill are quite popular, particularly among Pokemon GO trainers who love to battle, so Drilbur may get the shiny treatment sooner rather than later.
With so many events occurring on a monthly basis in Pokemon GO, shiny forms are being added to the game on a periodic basis. The game's community isn't quite sure how Niantic selects which Pokemon receive shinies and which don't, but the developers have quite a task on their hands attempting to placate aspiring shiny hunters. The number of Pokemon available in the mobile game is only growing over time, and shinies are worth looking at accordingly, at least within the constraints of the game as it currently exists.
For the time being, Pokemon GO trainers aiming to snag a shiny Drilbur will have to keep waiting and hoping. Since the year's calendar has almost come to a close for the game and Niantic, a whole new year of opportunities awaits players around the world.