Itis technically forbidden to speak the name of the Tetragrammaton due to the belief it was sacred.Only the high priest on Yom Kippur inside the Holy Of Holies in the Temple could speak it because of how Holy it was. On top of this the written version was lost when the second temple fell in 70 A.D meaning the NAME was lost completely over the next thousand years. Yahweh is no the actual name of G_D it’s just the closest men believe can be reconstructed from the hints we have left over. And even that is extremely inaccurate if one believes the theory it was 70 syllables long.
TLDR; It’s spelt that way out of respect to not use an inaccurate name for HaShem. Or at least that’s what I’ve learned from Jewish friends and biblical scholars.