Along with Smeargle, it has become quite obvious that Delbird (along with the Legendary Pokemon) are the only Pokemon currently not found anywhere in Pokemon GO. Unlike Smeargle whose issue revolves around its exclusive move of Sketch being implemented into the gameplay, however, Delibird already has a given moveset within the coding of the game. So why isn't Delibird in Pokemon GO yet?
Of all the rumors surfacing around about this, there are only two real reasons that could be the cause of this delay. The first has nothing to do anything with Delibird in particular, but rather with Niantic waiting to release Delibird exclusive to an upcoming event. Delibird is basically Christmas wrapped into a Pokemon, and many believe Niantic might wait until a winter event to release it. But since many believe the 3rd Generation will be released sometime during this summer, many believe Delibird will be apart of an earlier event (even if it doesn't really match) or just randomly like Ditto (and one day Smeargle).
The only other possible explanation for Delibird's current absence in the hit game is that Niantic is actually working on something similar to Smeargle's situation. Even though Delibird has a coded set of moves, it's extremely possible that Niantic only did this as they try to work out the mechanics involved with Delibird's Exclusive Move of Present. As you may recall, Present can actually do a huge amount of damage to an opponent or it can restore the opposing Pokemon's HP. As you can see, in a game based solely on damage, this can be troublesome.
Whatever the reasoning behind it is, players will just have to wait to get there hands on this Holiday-Themed Birdy. Luckily, it shouldn't be too long before both Delibird and Smeargle (and possibly the Legends too) are finally introduced into Pokemon GO.