I guess because it is functionally and visually indistinguishable from Fire Breathing at the moment?
I guess I'm just assuming that when he actually successfully performs Sun Breathing, it'll actually...ya know, look or DO something different!
Because if it doesn't, WHO CARES? It's JUST more Fire Breathing! Why should I care that Tanjiro actually does Sun Breathing if IT DOESN'T DO ANYTHING DIFFERENT?!
Seems like it's both. Tanjiro DID learn Water Breathing. But now supposedly his body isn't really "compatible" with it. Which is probably DNA related.
It's probably like using an element in Naruto or Nen type in HxH that isn't your specialty. You CAN use it, but you'll always be weaker at it.
That reminds me! I'm STILL waiting for this show to explain what the fuck sword color means!