Boomburst can be described as creating a sonic boom strong enough to break the sound barrier. In order for Flygon to create a sonic boom capable of breaking the sound barrier, it would have to be traveling faster than sound and moving approximately 750 miles per hour at sea level.
A better question is: Why can’t Galarian Zapdos or Kantonian Zapdos learn Boomburst?
Kantonian Zapdos should be able to learn Boomburst since a sonic boom is essentially the same thing as lightning and thunder. As for Galarian Zapdos, it’s confined to a limited area and can’t run 750 miles per hour at sea level or travel faster than sound.
Ummmm... if you look at the list of Pokémon who can learn Boomburst, absolutely none of them are known for being able to travel at supersonic speed and thus create a sonic boom induced shockwave via their speed... but they all (with only a few select exceptions) have designs/inspirations involving sound. Boomburst is also described as "The user attacks everything around it with the destructive power of a terrible, explosive sound," and is negated by the Soundproof ability, both facts further supporting the fact that Boomburst is simply an extremely loud and powerful sound generated by the user.
To go through every Pokémon capable of learning Boomburst, we can see that:
- Exploud generates Boomburst from its 'speakers' and huge mouth
- Vibrava, and in turn Flygon, generates Boomburst from the intense buzzing of its wings (also see Yanma and Sonic Boom)
- Noivern generates Boomburst from its speaker-like ears
- Kommo-o generates Boomburst from the loud clanging of its scales
- Rillaboom generates Boomburst from beating upon its wooden drum
- Toxtricity generates Boomburst from its guitar-like chest organs
- Taillow & Pikipek (who learn it through breeding) generate Boomburst with the flapping of their wings (although I can presume that Pikipek's evolved forms, especially Toucannon, may instead use their beaks)
- Chatot (also as a result of breeding) most likely generates Boomburst through its singing voice
There are plenty of moves that either grant or utilize their user's incredible speed, but Boomburst is not one of them. In fact, it's actually surprising Boomburst is not available to more sound-based Pokémon than those listed above.
Also... lightning and thunder are NOT the same thing, mate, nor are they identical to a sonic boom. -_-