Easy! Game Freak Logic
The game freak makes it to learn it, and with some reason.
But however, here’s my own opinion:
-Firstly, Dragon type are regarded as pure magic which Kygore has mention.
-Secondly, Look at this image:
Lucario wasn’t using its mouth to use dragon pulse, but it’s use it hand and do dragon pulse. Which means dragon pulse does not need any use of mouth, it’s own power could do it.
3. Being look more powerful
Lucario was totoally one of the most popular Pokemon, which makes me think maybe Game Freak like the design so want to give it looked very powerful.
However, these are only my opinion, ask game freak was more successful.
Hope it helps!